I have an account on bitbucket.org that I created by signing up with my Google account. Now everytime I log in I just click "Log in with Google" and that's fine.

How can I access my repos from git command line? It requests me username and password. I have a username, but no password. How do I log in then?

You should do a one-time setup of creating an "App password" in Bitbucket web UI with permissions to at least read your repositories and then use it in the command line.

How-to:

1. Login to Bitbucket
2. Click on your profile image in the top-right
3. Choose Personal settings
4. Under Access management section look for the App passwords option (https://bitbucket.org/account/settings/app-passwords/)
5. Create an app password with permissions at least to Read under Repositories section. A password will be generated for you. Remember to save it, it will be shown only once!
6. The username will be your BitBucket username (how to find it)