I am curious whether it gets more and more (legally) tricky for employees of large tech-companies to have side-projects. For example, if I am working at Google

I am curious whether it gets more and more (legally) tricky for employees of large tech-companies to have side-projects. For example, if I am working at Google/Amazon/Microsoft. What side projects won't get me into legal problems? Can I work on search, email, virtualization, etc. without being in conflict of interest?