I've had pools from 1.5' X 5' to 4' x 18'.
The last one came with a house I bought, but I moved the pool so I could build a deck from the back door to the pool.
Different pools have different set up instructions. It will say what kind of sand, if any.
Ring pools, also known as “float to fill,” are by far the easiest to put up. They get their name from an inflatable ring at the top.
If the pool is big enough, it will come with a filter and ladder. Until I built the deck, we put a kiddy pool at the base of the ladder for rinsing dirt and grass off feet.
If you get a big pool, get one locally so you will have service for the components.
If you get a ring pool, you can extend it's life with 3M 303 Aerospace Protectant (spray).
You will also need chemicals (chlorine is just one option) and a test kit for the chemical you choose to use. You will need the daily sanitizer and shock. And a floating dispenser.
You will need power for a filter. Possibly on a GFCI.
You need to check the laws where you are; some places require a fence with a locking gate.
You will need to check your homeowners policy - you may need to get a new policy.
And pool noodles. I don't think you can have to many. You can easily make things from them with cord and/or zip-ties. See youtube.
Glow sticks come in many sizes and are really fun in the pool at night.
If I had a pool now, I'd definitely also have bluetooth speakers - loud enough so they don't have to be close to the pool.