How to technically install a bathtub recessed into the floor

So I recently thought about sinking our bathtub into the floor so that instead of stepping over it, you step down into the tub....

Now I'm wondering how that is technically feasible or done.

When building, is some kind of basin poured into the floor slab where the tub is supposed to go so that there is immediately a recess, or how is that done?

And is that difficult to realize on the upper floor? Because if there are living spaces below, the tub can't protrude into the room in the ceiling, right?

What options are there that are also feasible?

Hello, I saw this about 2-3 years ago. A recess was directly incorporated into the base slab. But it is probably quite a bit of effort because everything has to be specially shuttered and a basket/reinforcement was also made. And the whole thing should be watertight afterwards. Besides, you have to know quite precisely where the tub should go. As well as how big/high the tub should be. There should be no problems with wastewater. However, I would not install the tub flush with the floor but leave a 15-25 cm plinth. It will surely be even more difficult at the top. But with sufficient construction capital, it is certainly doable.
Best to ask the architect.
Regards Thommi

We will install our bathtub directly into the screed in the attic, making it about 17cm deeper.