There is no default user. You set that up yourself, after installation.
If you’re looking for the URL to do that, it is /ghost, e.g. example.com/ghost.
That is documented here: How do I login to Ghost Admin?
If you’re with a managed hosting service, they usually send the direct link in a welcome email.
If you’re self hosting, the Ghost CLI (which is the only “official” way to self host – Docker is a community effort) also shows the correct path upon completion:
