NAL, but have heard of these. Amazon was really bad about it a few years ago.
General rule, anything you do on company time/devices/premises is theirs. That is enforceable for sure, and it’s provable.
Anything you do in your own time is generally unenforceable, unless you still code/materials or something from them. Caveat being that it can’t conflict with your work. i.e. if you work for Expedia and start a new travel company you may be in an enforceable grey area, “They stole company secrets to compete with us”. However if you work for a game company and make your own game, you’re probably fine.
Read your contract, know it upside down and sideways, that’s going to be the guiding principal.