How is unit testing testing anything?

I don't understand how I'm testing anything with unit testing.

Suppose I am testing that my repository class can retrieve values from the database correctly. The proper way to do this would be to actually call the real database and retrieve and check those values.

But the idea behind unit testing is that it should be done in isolation, and connecting to a running database is not isolation. So what is usually done is to mock or stub the database.

But why would testing on a fake database with hardcoded data and hardcoded return values even test anything? It seems tautological and a waste of time.

Or am I not understanding how to unit test properly?

Does one even unit test database calls?