They’re not occupying forces. There is a status of forces agreement between the two countries.

https://en.wikipedia.org/wiki/Status_of_forces_agreement

You are right. But it's a matter of perspective. In the mainstream perspective those bases are based on contracts and a method of mutual security. But there is indeed also the perspective in Germany that those bases are factually occupying forces and given their history the option of having those bases removed have been limited.

And there is a kernel of truth in it. The USA likely wouldn't give up Ramstein under any circumstances safe the German military mobilizing against them, the base is (was?) too important for the US. When Trump invades Greenland we will see this play out (how the base stays active and Germany is powerless to stop that).

So the occupier and country occupied signed an legal agreement making the occupation officially legal?

De facto and de jure are two very, very different things...

(not saying the US forces are occupying Germany, just commenting on op's logic)