Is it not a bit weird to freely give give away your entire code base (I assume it's personal, not your company's, but maybe I'm wrong) to an entity like Google?

As a business owner that uses Cursor, this is a real risk that I worry about (third parties stealing my code). However, the massive productivity benefit of having access to AI tools far outweighs the risk of them copying my business based on the code alone. Besides, AI is making code less and less valuable. My code is not the moat -- the hard part is the network, traction, brand, distributions, etc.

How common is it to have a personal project that isn't open source? Probably more common than I think, but it seems like a foreign concept to me.

Either my code isn't commercialized so I don't mind "giving" it away, or it is commercialized but wouldn't be safe from a clean room implementation anyway. Isn't that what bigco would do of they really wanted to steal your idea?