If you hire a new developer, are you comfortable with giving them your entire codebase?

I’m really curious about how this works.

Once you hire a developer for your project, they need access to the complete codebase. Doesn’t that mean they can just clone the project and replicate it, or just show it to a competitor...

If you make them sign NDAs, is it actually effective?

Is there a better way to handle this that I might not be aware of?

How do product startups typically manage to do this?