1972 house with 174 sqm from the neighborhood

You are acting as if you were buttoning up the parents' last shirt again. We don't even know that.

Thank you, not a word has indeed been said about the total assets yet.

Thank you, there actually hasn't been a word said about the total assets yet.

If they had money to give away, they wouldn't lend it, but rather give it to their son as an early inheritance.

If they had money to give away, they wouldn’t lend it, but rather give it to their son as an early inheritance.

That is not correct .. my parents also granted my brother a favorable loan for purchasing a house, but indeed a LOAN, and it was fully repaid. They could have given it away, even without stress with me. But, LOAN was already a condition on my brother’s part. A loan at this point is simply often a joint business between parents and child. Because, whether today with partially custody fees for larger sums, or 15 years ago, the discrepancy between interest for invested money in overnight or fixed-term deposit accounts and the interest to be paid for mortgage loans exists. And, if the family agrees on the average rate, both sides win.

Ditto with me

(Addition of characters and words without discussing their pointlessness)

Well, the OP has remained silent about the situation and the open questions since I brought it up.
I read from this:

craftsmanship is present (I renovated my grandfather's apartment and my parents' apartment together with my father and grandfather -

whether concreting, laying screed, electrical work, heating, water, sanitary, tiling, floors, windows or doors ... I was involved, we did most of it ourselves or with the help of acquaintances and friends from the specialist fields and we were assistants)

I had 2 part-time jobs during my studies and sacrificed my entire free time for 2 years to help my parents renovate the apartment - I tried everything that was possible within my scope)

From my parents, I would get an interest-free or low-interest loan from the savings for their retirement provision to improve the equity

My father told me that if I really wanted it, and a few ten thousand euros reduced the interest by one or two tenths, we could manage that.

But I do not assume that we are talking about a very large amount (maybe 50-70?)

... that money is not exactly so liquid that the parents just have money left over.
The OP is talking about a loan and not a gift.
It may be that for some it looks as if the parents are a method of upbringing – for me it reads as if one has to live frugally in order to have a little more later. And not “a little more” in order to indulge even more. The reason for the interest-free loan could be gratitude. All understandable.
But it is what it is.
There is no need to sugarcoat this here – unless the OP says how it could be more positive.

If they had money to give away, they would not lend it but give it to their son as an early inheritance.

No, if, for example, there are other children, a loan is a simple and fair option that does not directly disadvantage the other children. Alternatively, there are families that play games like child 1 gets money in advance, child 2 inherits the old house. My father-in-law also gave us an interest-free loan, even with the addition that it lapses upon death and does not have to be repaid. It contradicts the fairness from above, but he is a stubborn old man ;)