A large percentage of U.S. software jobs (and probably nearly all YCombinator startups) are in California. Other states might be different, but stuff you do outside of work doesn't automatically become your employer's IP in California.

There are some nuances and I'm not a lawyer, but the gist of it is that three ways to trigger the IP to attach to your employer:

1. You do it on-prem or during work hours (but work hours are flexible for salaried employees)

2. You do it using company equipment (say, company laptop at home)

3. It's reasonably related to what you or other people do at your day job

If none of those apply, then you own it. That's relevant to the discussion at hand because, at least in California, you could work from home for two companies with unrelated businesses and not break any rules.