Exit terrace doors and external window sills, floor-to-ceiling windows

Make a simple sketch (cross-section) of your plan. Normally, granite slabs have drainage grooves on the left and right and a continuous groove at the bottom as a drip edge. We also have such slabs. These are laid with a slight slope. I am attaching two pictures of mine. Unfortunately, I don’t have a picture before the exterior plaster where the step is fully installed. In my pictures, you can still see an aluminum rail running lengthwise to which the granite slab is butted from below. That would still be missing in your case as well. Definitely do not use polished benches! The exit steps must also have a minimum distance between the finished facade and the outer edge – so they shouldn’t stick out too far. Can the plasterer not do this?

the other two are without a door.

So basically fixed as well.

Two of them are patio doors + fixed glazing,

I would pave up to the patio doors with fixed glazing or place the mentioned granite blocks. For fixed glazing, I would have a window sill installed corresponding to the material you are planning in front of it (pathway or gravel or lawn). Of course, everything should be done so that nothing rubs (stud membrane etc.). With exposure to rainwater, a drainage channel would probably need to be installed with paving?!

15cm distance without a channel. With a channel, I believe it is 5cm according to the standard. We are installing a channel in front of the exit step. Since we do not have threshold-free floor-to-ceiling windows, we come to about 5cm between the frame profile top edge and the terrace top edge. I would also suggest the same benches everywhere. Thickness from 3cm. If the terrace is to be lower, then the 15cm block step.

The plasterer would then plaster that. Before that comes a dry bed as a drainage strip. I initially thought I would do it after the plasterer is finished. But if he plasters it right away, that's all the better and I don't have to worry about the transition.

But that only works with fixed glazing. You can't put a bed in front of the doors. I would find a kind of windowsill, as you intend, too small (for the doors). You need a minimum tread depth of 30 cm, better more. Then you wouldn't even have half of that bearing. I would be afraid that it would break off to the front. Or am I misunderstanding your plan? I would rather take steps like Nida35a, then you don't need to have anything plastered and you have a good tread surface. That's how we have it.

everything sealed with Flüssigfolie

Wtf is "Flüssigfolie" supposed to be? - "Folie" implies to me a material in the solid state.

He probably means liquid plastic waterproofing. What the roofers use when they don’t want to use flames because there is something else sensitive nearby.