> would your employer care (if they found out) if you copy/pasted a small portion of code that’s not considered critical IP

Who decides what is "critical IP" and what is not?

This is a terrible advice, and no, you should never copy code from a work project without being pre-authorized to do it.

It's not just common sense, but you also might trip up over unexpected legal issues (licences etc) that you may not even be aware of.