Are they putting their cash into gold or similar then? A market crash will also coincide with the US dollar crash, no? Or is a market crash separate from the US dollar failing?

A bubble burst or similar doesn't have to crash the whole economy. It definitely can, especially when it's stuff like financial or real estate. A tech bubble burst would devalue many of the highest valued stocks but doesn't have to take the entire USD down with it.

That being said, I haven't studied economics and am very much a layman. So if someone wanna correct me, please do!

There's even a precedent. https://en.wikipedia.org/wiki/Dot-com_bubble

Sucked for California. Everywhere else was fine. But traffic was so much better in the bay so there's that silver lining...

If Berkshire Hathaway sells 1 million shares of AAPL for N dollars per share, and the market drops the price to .8 * N dollars per share, he just buys more shares back with those same dollars, for example.

It would be a very Buffett thing to do, because shares produce dividends per share, so increasing share count is its own strategy.

They are not necessarily correlated - but often can be.

[https://www.fidelity.com/learning-center/trading-investing/s....]