First of all, I searched for info about this on travel.stackexchange and various websites but I can't seem to find a definite and unambiguous answer.
I'm an Afghan citizen traveling to Finland and my flight has a stop in Germany. I have a Finnish residence permit for study, but I haven't entered Finland yet so there is no stamp or anything on my passport.
Generally speaking, as explained in Do I need a visa to transit (or layover) in the Schengen area?, holding a residence permit from a Schengen country exempts you from all visa requirements. In that case, you will enter the Schengen area in Germany so what you would otherwise need is a regular Schengen visa (which can be issued for transit purposes), and not an airport transit visa.
However, if you have a regular residence permit (it looks like that), it should be fine to travel without visa. You don't need any stamp or sticker in your passport. You simply present your residence permit together with your travel document (I don't have one personally but I have travelled with friends and colleagues who do). You will get an entry stamp from the German border guards.
