Cracks in window sills normal?

One can, but does not have to install window sills and doors before the interior plaster/screed. Both are common practice and have advantages and disadvantages.

Are we talking about the door leaf or the door frame? Door frames must be installed before the interior plaster. The interior plaster abuts the door frame...

We are building turnkey with a small construction company from the local area. The trades such as electrical, carpentry, and plumbing have been awarded to local companies with whom they have worked for many years. So we really cannot speak of a cheap provider, everything planned by the architect and the construction management is handled by our house construction company. I also believe that the cracks are not normal, although I have also heard opinions that this is normal for a natural product.

I would also say that the frame elements should be installed before the plaster, because the plastering should be as uniform as possible. It can be done differently, but then it has to be patched afterwards, which always carries the risk that the "seam" will be visible.

We are building turnkey with a small construction company from the local area. The trades such as electrical, carpentry, plumbing have been awarded to local companies with whom they have been working for many years. So we really cannot speak of a cheap provider, everything planned by the architect and the construction management is handled by our house building company. I also believe that the cracks are not normal, although I have also heard opinions that this is normal with natural products.

When I receive my 8m natural stone kitchen countertop, I do not necessarily want it to have cracks. That would clearly be a defect. It is the same with window sills, size does not matter.

Are we talking about the door leaf or the door frame? Door frames must be installed before the interior plaster. The interior plaster directly adjoins the door frame...

No, door frames are always installed after the interior plaster. Frames only if installed flush.
You can clearly see this in practice when you enter existing buildings and notice that there is always plaster under the frame.

If you install before the plaster, you'd also be installing before the screed. Therefore, you practically cannot install the door frame already, as it is set with a minimal gap to the screed and the floor covering.

When I receive my 8m natural stone kitchen countertop, I definitely don't want it to have cracks. That would clearly be a defect. The same applies to window sills; the size doesn't matter.

:) And now everyone reading here knows that you are planning an EIGHT meter long natural stone slab in the kitchen. ;)