Which ground is suitable for a prefabricated garage?

No, the garage has no floor. The precast parts are then sealed towards the floor.

In this case, only a slab foundation would be an option for me personally.

a steel wall garage that is plastered and thus looks like a solid garage.

My landlord himself was a manufacturer of garages and halls and did not offer plastering, but only sand coating - presumably he shared my skepticism about plastering steel. The structure can keep up with plaster - also in terms of lasting appearance - except that the material does not "reveal" itself through the groove. However, it must be said: there is a pumice "prefab" garage manufacturer here in town and a (then two) concrete garage manufacturers in the surrounding area; perhaps the small price difference between a high-quality (and thus more elaborate) plastered steel garage and a concrete prefab garage made it "practically forbidden" to offer both in parallel.

In this case, only a floor slab would be an option for me personally.

then two worlds collided, insofar as the steel garage is also valued for the advantage of easy relocation.

It is definitely not planned to move or enlarge the garage in a few years. It will be built directly on the boundary and represents the maximum size.

In my opinion, a concrete slab belongs underneath as well...

two worlds then collided, insofar as the steel garage is also appreciated for the advantage of easy relocation.

A garage is not seasonal decoration. It is installed and remains in place.

In my opinion, there should also be a base plate underneath...

Haven't you actually answered your own question then?