A lot of companies were founded by one or a group of people leaving a company and opening a competitor with the knowledge they gained while working for them. Anthropic, as I understand it, is an example (from OpenAI). Even the company I work for was founded by people who used to work at the "old" industry giant and today we compete with them directly. This is not criminal unless you signed a contract forbidding doing that, though you may certainly claim it's morally a gray area. But yeah, it's very common. You don't get lots of domain knowledge, which is required to start most kinds of businesses, until you've worked in the area for someone else for a long time.