Acquisition of a vacant farmstead in the outer area

Sicht-Estrich means you can see it? Nothing will be put on top? I wanted it that way for our underfloor heating as well, but my wife wanted wood. Now it’s thin, dark oak, looks great and feels good.

Exactly, a trowel-smoothed and then PU-sealed residential concrete floor with very few open joints probably throughout the entire house. It was very difficult to find a good provider for this and I am very much looking forward to the result.

oh ... and how are the corners made?
Tongue and groove "isn't either"?

Which corners do you mean exactly now?

What I "always" like is when the material from which something is made can also be seen. That has something "honest" about it and helps (at least that's how I feel) to understand the world. In the case of exposed screed, it seems to me that the most important thing is how the surface actually looks, because: repainting is definitely NOT what is wanted here. There are probably quite a few possibilities. (When it comes to cement, I always refer to Sven Backstein and his "Mörtelshop" (with oe). He knows his stuff well and provides detailed and quick answers.

For example here, the corner between the window and the wall

oh ... and how are the corners done?
Tongue and groove "isn't it" either?

Tongue and groove could have been had for much more money as well. We have also looked at it with other builders. I find our formwork much more authentic - especially because it is not so perfect. It was supposed to be a residential barn. In the outside corners, I use strips made from the same wood. Some are already in place.

Tongue and groove could have been had for much more money.

If that is not technically necessary, then it is not necessary.
Tongue and groove is probably more stable overall (warps less because the boards hold each other), less air gets through the cracks, and if the air is dry and the cracks are therefore wide, you cannot look through the cracks.
But your solution seems stable enough to me anyway, and wind will whistle little behind the boards ;)