Who will build a massive garage (3.50x6m) out of bricks - who? (Brandenburg)

What exactly is expensive about a garage? The foundation (slab)? The walls? The roof? The door? All of it? Is there any reason against doing it yourself? I suspect that the previous owner of my property built his two garages (sheds) himself from concrete and old bricks. They held up perfectly (the demolition excavator took almost longer on the garages than on the house) and I doubt that the material costs were more than a few marks. However, it can be assumed that he invested quite a lot of working hours.

What is actually the expensive part of a garage?

Honest short answer: the price. Seriously. At least with a single garage with a swing door.

May I understand it this way, that a garage actually should not cost 20-30 thousand euros, but it costs that much simply because it is paid for? That if one could, self-construction would only cost about 1/3 to 1/4 of the expenses?

Not that dramatic after all. But the car is the German's beloved child, for which nothing is too expensive. That is why garage prices were already before the current construction boom as if the value-added tax had been accidentally applied twice (and since the beginning of the construction boom like three times). Stones, on the other hand, cost the same, whether for partition walls or for garages.

Could it possibly cover the majority of the costs if one builds most of it oneself with foundation construction and wall construction using YTONG blocks, which could potentially be done by a layperson with a lot of time and skill if they study extensively (no, I do not want to disparage the craft, but I could imagine that especially for the foundation and walls the time/work effort is the cost factor). Then, one could specifically hire individual companies for the window, the door(s), and the roof, as these trades might possibly be the most difficult.