What if… (some questions about ETF investments in particular)

I have been investing in ETF’s for the past 4 years, and while I feel it is the safest and easiest among other investments, I still have some concerns that might be illogical to many. But the past years had shown us that illogic is probable to happen at any time (example: who would have guessed that cities and counties would get locked down like it did in 2020?).

I also understand that diversification is the key, but what exactly do you diversify? The ETFs? The currency? The market? The investment (etf vs real estate vs gold)?

I would love to read some comments (reassuring or not) about the following:

1- we always hear “VTI and chill” but what if VTI (or others) deems to stop existing

2- what if Vanguards (or Ishare or others) collapses

3- what if the brokerage (IBKR or others) collapses

4- what if the dollar (or any currency) lost its value

5- what if the stock exchange or stock market collapses?

… and the list goes on…

Be harsh. This is better than waking up one day with a huge loss.