For accuracy's sake - it is not about citizenship but about where you have been recently. A Belgian citizen who has not stayed in a risk area recently can enter Germany and stay in Germany for any purpose without quarantining. A German citizen coming from Belgium would have to quarantine.

Someone coming from Belgium can enter Germany from anywhere for the purpose of transit without needing to quarantine either, but they have to leave Germany by the most direct route. They can also enter the areas of Germany bordering Belgium (i.e. NRW) for short trips for various purposes, and furthermore the quarantine obligation in NRW is not currently in force (but you must still register your entry except for certain short trips and you may be obliged to take a COVID test, which if positive would require isolation).