#1&2. What assets do you invest in?
I base my investments on three major groups. What amount would my family need to weather a short-term catastrophe (loss of both jobs, major natural disaster), to weather long-term loss of income and what's left over to speculate with.
a. We keep enough money for 6 month's basic necessities in the bank. It is a small fraction of our overall portfolio and the main priority is immediate access.
b. With that covered, we max out our 401ks with bond funds to pay for basic essentials for the family. The goal here is $1.5 million in 15 years (dual solo 401ks) to generate at least $30k/year in interest. This can also be passed to our children tax-free in event of our deaths.
c. The bond investments alone will probably be ok for early retirement, but there's savings left over that we invest in stock index funds. That's for retirement discretionary spending. I make no assumptions about what will happen to it other than it will probably increase over the long run.
As you can see, our most risky investments are in stock index funds, which in the long run are acceptably safe. But again, we don't plan on necessarily having all that money available immediately. Between the bonds and the cash, we should be able to weather some period of stock crash/recovery.
#3. Please give me an understanding of how you would do things differently, with regards to investments, if you started over at 25. No assets, no liabilities. Making 50k a year living in a cheap part of the US.
Earnings-wise:
The biggest expenses you'll face at this point are education costs. Don't overshoot! Get a very good understanding of what a degree you may be considering will get you. I know too many people with huge college debt they have a hard time servicing because they thought it would be useful or interesting, but no one hiring shared their opinion. Also, incredibly important, prestige of your college will probably not matter much in the long run for most professions.
Savings-wise:
I recommend to keep a few months' cash in the bank to weather short-term crises. Then, consider investing in stock index funds with some smaller amount of bond index funs. For the long-term, try to set a timeline (i.e. retire in 20 years) and two goals to reach in that time: a stash I can subsist on without a job (retirement or laid off), and one I can use to occasionally spend on luxuries. If your subsistence level can be achieved with just bonds, then you can use your stocks for the extra spending. If the stock market drops 50%, just spend 50% less on random stuff. Your baseline quality of life should remain fine. To figure out the amounts needed, consider that one can withdraw about 4% of a given stash over several decades with a good chance of not running out.
As you get closer to retirement, and the psychological importance of having a baseline income increases, you may adjust your percentages towards bonds. With several years of experience (and hopefully more income) you should have a better understanding of your preferred allocation and retirement budget.