Backfilling or basement construction?

Why? No idea, it's definitely nothing special:

Then you must have a cheap shell builder – with the roof covered I would be around 160,000 EUR?

The basement was broken down to about 60-65,000 EUR – so bare. But shell height 250 cm and rather designed as a living basement (including landing staircase and second staircase to the garage).

Shell height 250 cm is only about 235 cm finished, right?
I think we have just under 230 cm including tiles. 60-65,000 for a bare basement is quite steep, although it also depends on the size...

236 cm - but did I say something different there? In the offer above me, the shell height was 225cm...

There is nothing tight about that - it's just a normal price. But the basement is not itemized separately - you have to deduct it.

The basic dimensions are 11.5 x 9.5m.

Now there is underfloor heating everywhere and everything plastered and electrical wiring concealed plus a shower bathroom.

Ok, but the interior design was definitely extra?
Ours is also smaller (just under 70 m²) I think.
Does it perhaps matter whether it is brick or concrete? Even if it is not a [Weiße Wanne]?

No, nothing should happen - ours is concrete with waterproofing and perimeter insulation.

And of course, that's all extra - the structural builder doesn't do that.

Ours is brickwork (30cm KS plus concrete earth pressure beam), bitumen waterproofing, wall protection panels (about 7cm Styrofoam) and dimpled membrane.

For the price of your basement, some would build a 90m² bungalow.