By way of your citizenships you’re admissible to both Germany and the US and can do the trip assuming you find the means of travel that works for you to complete your journey.

In Germany the rules vary by region where you’ll be in Germany; and in the US they also vary by region where you’ll be staying. At least in the US, there is — with some exception — generally no enforcement to make sure you do what is asked of you (in terms of quarantine/isolation) once you’re admitted back into the US.