My wife is a Bulgarian citizen and she wants to move to Germany or Austria. What is the procedure in order for me to join her? Can we apply upon arrival? We both hold B2 certificates in German.
As long as she is entitled to free movement (e.g. working, more details here) then you are entitled to live with her. The procedure is much more flexible than it is for other foreigners or for the spouses of German citizens so you can indeed travel together and apply upon arriving.
Unlike a Serbian citizen coming on their own, the spouse of an EU citizen doesn't require to secure a long-stay visa before entering the country and can transition from a short stay to a longer period of residence, completing all required formalities without leaving the territory.
In practice, you can simply apply for the residence card at the same time as your registration or immediately afterwards (see e.g. the procedure in Berlin). Speaking German is great and will help you but it is not a requirement for the spouses of EU citizens.
I haven't done it recently but in my memory, the registration of your place of residence (Anmeldung) is processed immediately, you leave the building with the confirmation in hand. You might need a separate appointment with another office but you can use that document to apply for the residence card without any wait.