I'll bite, although I don't know how enlightening this will be.
42yo female, never overweight, in fact, often feel underweight. If you look at photos when I was a child and teen, I was skinny, skinny, skinny. In college I was accused of being anorexic. I have never had any issues with or around food or food consumption. I strongly suspect genetics plus environment with a dash of luck in my situation.
Family
Both my parents and multiple (most?) extended family range from husky to downright obese. My parents are overweight (my mother may be obese at this point). However, my grandfathers and one grandmother were thin, and the one great grandmother I met was also very thin. My sister is more muscular than I am but not husky, so I would consider her on the healthy side of thin. I am extremely slender at ~ 115-118 lb and 5'6" - it's the very bottom of the "healthy" range on charts, and I often feel like I would look and feel better if I gained 5 or 10 lbs. Sometimes I wonder if there is something wrong with my thyroid, but I've never been tested (I sort of avoid doctors).
Diet
I am vegetarian and have been so for ~ 15 years (but was the same body type/weight prior). (My sister is vegan and has been for more than 25 years.) I *love* vegetables, and they probably make up the bulk of my diet. This results in me consuming fewer than ideal calories sometimes because they are filling, generally. I don't limit carbs in any way (although I am considering looking at refined white flour as sometimes I feel terrible and tired after eating a lot of bread/pasta). I don't limit sugar, in fact I won't eat/drink sugar free or fat free stuff because I don't like the way it tastes. When I was younger (high school, college), I drank a LOT of regular soda. Now I drink it only occasionally but I drink a LOT of coffee - with cream and sugar. My sugar intake is probably lower now than when I was younger. I have occasionally tried tracking calories to make sure I am getting enough, which is a little tricky as I, like many people are reporting, only rarely eat processed foods. When I do track, it is pretty obvious I often need to increase calories, which I usually try to do with oils, avocado, cheese, sometimes more carbs, but it can be really hard to hit 2000 calories sometimes. When tracking I often hit around 1500-1700 naturally, and then have to work by adding things to make up the rest. I make a lot of my own food, including bread usually, yogurt (plain), granola, vegetable stock, seitan, sometimes pasta, sometimes ice cream or gelato. I feel like I have a high metabolism, and a small stomach, as I feel full pretty quickly. I do graze, because if I waited for "meal times", I would be starving. The one exception is that I can't/don't usually eat breakfast until 10am or later (except for coffee in the morning) because I am simply not hungry and if I eat earlier I have to really force it. So I probably go 10-12 hours overnight not eating. However, during my daytime hours I do eat regular meals, plus snacks in between, and sometimes dessert or nighttime snack before bed. Also, if I don't eat something every 2-3 hours, sometimes I will feel jittery or shaky. I should probably get that checked out. I don't have a lot of self control when it comes to a few specific things, like kettle cooked potato chips (mmmmmm), so I usually just don't buy them. When they are in the house I can eat a bag in 1 - 2 days, and this wreaks havoc with my digestive system, and also sometimes my skin (which is generally, typically, quite good with no special effort). I do have a sweet tooth, and if sweets are offered I will definitely take advantage. I really enjoy cooking and I like trying new things. I like food from a large range of cuisines and will sometimes settle on one for a while before moving on. Like I said, I am vegetarian, for dinner I eat fish about once a week, a bean-centered thing about once a week, sometimes lentils, tofu about once a week, sometimes seitan. I eat eggs and cheese (not as much dairy as I used to, except homemade yogurt and half and half or cream in coffee, occasionally homemade ice cream). I do not eat peanuts or tree nuts as I am allergic. I do drink occasional alcohol, (maybe 2-4 servings per month on average?) everything from mixed drinks to wine and hard cider and sake. I don't drink beer (don't like the taste).
Note: When I was growing up we also ate *mostly* non-processed foods. My father cooked dinners every night consisting of mostly whole foods (with the occasional boxed scalloped potatoes or shake and bake seasoning). We always had a protein (meat), starch (rice, pasta, potatoes, bread), and vegetable (cooked or a salad). Examples: pork chops with canned pineapple and rice. Italian beef and noodles (ground beef, egg noodles, tomato sauce) and green salad (lettuce, cucumber, tomato, carrot, dressing). London broil with baked potato and cooked broccoli. Shake and bake chicken with mashed potato and cooked carrots. Pasta with meat sauce and garlic bread and salad. You get the picture.) My parents were *not* health food nuts. As kids we usually drank milk with dinner, and often other times (cereal with milk for breakfast on weekdays). Nearly my entire grade school career I took bagged lunches that consisted of a tuna sandwich (because I disliked lunch meat), a fruit (apple or orange), a veg (carrot sticks or celery), and 2 small chocolate chip cookies. My parents would cook breakfast for us on weekends too, eggs, or pancakes, or waffles. My parents had a relatively small rotation of meals they went through (maybe a dozen?) but we rarely ate fast food or out at restaurants. We were not denied things like candy, but we had to buy it ourselves if we wanted it. This likely informed my habits later. When I was in college, I ate in the dorms for 2 years, then I lived off campus for 2 years. I ate a lot of pasta with vegetables, tuna sandwiches, cheap cuts of meat. I occasionally cooked things I had eaten at home. (As an adult I cook a huge variety of types of food, so many more than when I was young. Still, being able to cook decently healthy meals to start was likely helpful.) I did drink a lot of soda in high school and college, like I said. I also ate a lot of chips. We almost always had dessert (ice cream usually). For my own life and eating, I believe everything in moderation (so, nothing in particular is "bad"). I eat butter. I eat full fat dairy. I eat salt (but mostly what I add, because, again, not a lot of processed stuff). One thing I do notice - many people in my extended family say they *crave* bread, and eat a ton of it. I don't have this craving, though I enjoy good bread as much as the next person, and I usually bake my own.
Activity Level
I did dance as a child, 3x a week, and in high school I also did student theater 1/2 school year (including usually being cast as a dancer), but no "sports". Keep in mind this was at a time when computers and smart phones were not so ubiquitous. I'm Gen X - I grew up with computers, but internet was slow and games were not addictive as they are now. I spent a lot of time outside as a kid. I read a lot (introvert), but also we would wander in the woods behind out house. We didn't do a lot of hiking as a family (that came later).
I was a competitive ballroom dancer in college, which meant practice for 2 hours 3x a week - all of that was not dancing though. We also sometimes went to social dances. We competed maybe 8-10 weekends a year - this was a LOT of dancing, but in short spurts. I didn't do sports other than this.
After college I had a lot of jobs that included heavy field work (carrying heavy backpacks, walking miles and miles, physical stuff). I also started doing swing, and at the height I did classes maybe 1-2 times a week, plus social dancing 2-3 times a week. This is also not constant, but I am sure a *lot* more activity than you "average" person.
I've had a 99% sedentary desk job for the past 11 years now. I have recently tapered off dancing to nearly nothing. However, I now have a garden that is a lot of work, and a yard with grass that has to be cut and weeds that have to be pulled. I sometimes joke that I don't understand why people go to the gym, since yard work is really a workout, lol. I also like to hike, but the frequency and amount is rather sporadic. I have a dog (2 years now) that I take for walks usually 2x per day, usually about 30 minutes each time; we only go local now, it's flat, but occasionally fast (she is old but still likes to run a bit; I can't hike with her now as she is too arthritic at this point). Sometimes we skip cold nights.
I *feel* very unfit (like, skinny fat), especially after these many years of the desk job and tapering dance off to almost nothing. I am considering starting pilates maybe for strength and flexability, which I used to have but don't anymore, as I get older. My diet and exercise level have NEVER changed how much I weigh - I might fluctuate a few pounds here and there, but I can't gain weight to save my life. It has only affected how I feel my FITNESS level to be. Even now, I don't *feel* like my activity level is very high, yet clearly it is higher than the majority of people. Oh, also, I pace a lot even when at home, I have a lot of nervous energy. Though minor I'm sure those burned calories add up.
(Also, ancillary data - my sister, the vegan, has a physically demanding job, she is a vet tech and regularly has to lift 50lb dogs. She also takes her dogs on walks and hikes and kayaking on weekends. I'm sure she would say she doesn't exercise (because she doesn't run or bike or go to a gym), yet she does a lot of physical things. So her lifestyle is a lot different than mine, but she (as far as I know) doesn't really regulate her diet aside from being vegan, and I know she doesn't really have a sweet tooth the way I do.)
I'm sure there are things that I've missed, but that's about it.