Interior walls made of drywall or masonry?

With two GÜs we negotiated with, everything made of masonry (sand-lime brick) would have been possible for a small extra charge,

Incredibly, I can't remember right now whether you have a straight-walled upper floor or a sloped attic.

We have a straight-wall upper floor (and a straight wall attic ;) )

A concrete ceiling only goes from the ground floor to the upper floor. For drywall partitions, the general contractor (as mentioned by ) suggested working with supports.
A concrete ceiling from the upper floor to the attic is not permitted due to the development plan.
Here we would install a drywall ceiling.

And yes, this is all new to us, which is why I like to follow along here ;-)
One or two questions will certainly come up again during the construction phase.

A concrete ceiling from the upper floor to the attic is not permitted due to the development plan.

You see me completely confused. Which development plan prescribes the building fabric?

For drywall partitions, the general contractor (as mentioned by @K1300S) suggested working with supports.

That is unsatisfactory: the supports protrude due to their cross-section and only transfer the ceiling load but do not provide bracing. Supports belong in skeleton construction and have no place in a single-family house.

A concrete ceiling from the upper floor to the attic is not permitted due to the development plan.

I fully agree with my predecessor ;-)

You can also take a look at [Gipswandbauplatten] - we have them for all interior walls.