Install a drywall partition for the bathroom

In this forum, you have to expect anything.

It is practically teeming with professional expertise.

So clear statements are necessary. And a circular saw is simply not a handheld circular saw.

A cordless screwdriver is not a drill press even though you can drill with both.

Please what? Are you trying to mock me?

Circular saws are divided into stationary and handheld circular saws, which are each further subdivided into subcategories (table, swing, miter, / plunge).
So a circular saw can very well be a handheld circular saw.
A handheld circular saw is always a circular saw.
The statement a circular saw is not a handheld circular saw is simply false. Possibly a lie (to avoid admitting that one just wanted to know better), but definitely wrong.

Make up! Another important question for me: the dimensions of the walls are 2.6m wide by 3.4m high. I have OSB boards measuring 205x67.5cm and GKP measuring 120x60cm. Did I choose the GKP too small? Does this have any negative effects on the stability of the wall?

You lay OSB first horizontally (groove down) from bottom to top. Then the GKP vertically. It should not actually happen on the way that you have too little offset with the joints. Maybe lay it out and have a look before (the GKP), otherwise one correction cut.

Yes, one could certainly have chosen larger drywall formats. 2.60 x 0.6 or even better 3.60 m high. You just have to do more filling because of more joints. You also have to be able to transport them.

Would you first completely cover the ceiling with gypsum fiber boards and then attach the CU profiles on a continuous surface, or screw the CU profiles onto the wooden battens, build up the wall, and only then cover the ceiling after two rooms have been created? However, as far as the wooden battens are concerned, I cannot achieve a continuous surface, meaning the CU profiles would only lie on a batten every 30 cm and there is space in between.