It probably doesn’t matter in practice. They will have more lawyers and more money to burn on legal process than you. Who will go bankrupt first fighting a legal battle between you and, say, Apple?

In practice, I don't think I've ever seen a SV example of that kind of litigation from garage development of IP. Maybe I've just missed them?

Theft of IP from one company to a new company on the other hand there are multiple public examples of.

The only time they'd actually sue is if you made something worth money. in that case, there's a good chance you can get investors to pay the lawsuit.

Investors would check if you were subject to an IP assignment clause in your employment contract prior to investing. It's part of basic due diligence. They would then most likely pass on the investment.

It does matter in practice in California.

If its "related" to your employers business yes - unless you agree a variation of the contract.