i spent three years working on a small SaaS on evenings and weekends while at a big tech company. after i left, i actually sat down and read the IP assignment clause in my old employment agreement front to back. it was not pleasant reading.
here's the rough breakdown of what those contracts usually say, at least in california:
the dangerous part: most big tech employment agreements have an "inventions assignment" clause that says anything you create during employment that relates to the company's business OR uses company resources is theirs. "related to the company's business" is written so broadly it could cover half the software universe.
california carve-out (Labor Code 2870): california actually has a statute that partially limits this. if you develop something on your own time, with your own equipment, and it doesn't relate to the employer's current or reasonably anticipated business, and it doesn't result from your work at the company, you keep it. other states mostly don't have this.
what "company resources" actually means: signing into your personal github on your work laptop. sending a test email from your work email to debug something. one afternoon of using your work computer "just to finish this one thing." all of that potentially taints your project.
what i actually did for my project: personal laptop only. always. personal github account, separate from anything work-related documented start dates in git commit history (helps show it predates certain features at my employer) kept a short notes doc logging that i worked on it evenings and weekends, not during work hours
the honest answer is most employers won't come after a side project that isn't making serious money and doesn't directly compete. but "probably won't" is not the same as "can't." and if your project takes off, you really want clean title.
if you're building something and you haven't read your employment agreement: go read it this weekend. not fun, but necessary. if the language is genuinely broad and you're in a non-california state, a one-hour consult with an IP attorney is cheap insurance.
i'm not a lawyer, this isn't legal advice, etc. but i wish someone had walked me through this before i started.