I have almost no experience in TDD and have to use it in a uni course where I build an Android app connecting to Firebase. I have googled for I don’t know how long and found no examples or got repos with unit test using Firestore.

(Unit) testing Firebase (or any other 3rd party service) does not make much sense - they should be tested by their own development teams.

What you should be unit testing is your own application logic, using mocks or similar replacements for any external services.

Use mocks to return positive and negative results you can test against.

Integration testing is what it sounds like you're trying to do, which you would just set up an automation suite to test you're code and the integration to other services.

TDD is just programming with extra (painful) steps. And if your professor doesn’t agree he can fight me.

TDD is quite honestly the biggest waste of time and utter bullshit I’ve seen in a while, it backs you into a corner, it makes refactoring way harder down the road, and it sucks up valuable development time. It only ever makes sense for situations where you have a very prescribed set of inputs and outputs.

Feeling for you.