S&P takes the 500 most profitable companies in the US and builds an index off of that. S&P usually goes about 7% return but markets are very bullish on AI. It's similar to the dotcom bubble so definitely not without risk.
Currently nvidia is driving the S&P up but nvidia might crash one day like cisco did back then.
I concur on this one. Open a Roth IRA and put it towards an S&P 500 mutual fund with low fees. I use Schwab because it's free to open an account and deposit money.
Strategy wise it's good to mention that you approach this as a savings thing so you deposit, say, monthly. That way you compound your interest and that can really ramp up quickly.