Buying index funds (either mutual funds or ETFs) has been an effective approach for retail investors. But the concern now is that some US stock index funds are so heavily weighted to the "Magnificent 7" stocks that much of the previous benefit of diversification has been lost. The Mag 7 are all highly correlated with each other so if one falls then usually the others do as well.

https://www.fidelity.com/learning-center/smart-money/magnifi...

There are other index funds which are equal weighted rather than market weighted. Those have underperformed lately but might be less volatile if the AI bubble pops.

I'm in a global tracker and the sheet amount in these big stocks is scary.