Answers to that depend on state employment law -- California has an explicit proviso that employment contracts can't claim certain work the employee did on thei

Answers to that depend on state employment law -- California has an explicit proviso that employment contracts can't claim certain work the employee did on their own time, with their own stuff. But most states don't, and even California's provision has limits -- IIRC, the project has to be unrelated to the business, and what's unrelated to Google's business these days? Or Amazon's? I've gotten explicit waivers from my employer for unrelated projects, which hasn't been a problem. But people who just don't tell the boss run the risk of legal action if they find out about it by other means -- so, at the very least, I'd suggest a chat with a lawyer well-versed in your state's employment laws before working on any side project of potential commercial value, with your employment paperwork in hand (especially the Intellectual Property assignment contracts, or their equivalent), so you have some concrete idea what risks you're running.