Construction costs are currently skyrocketing

If you have the money, or it’s not needed elsewhere, you can do that, yes. Some do not have that luxury.

I do consider myself capable of "helping" with something like that, but starting from zero to "just do it" is quite scary for me with many things when we have never done them before. Especially when some devices are involved that I have never held before. And then there's the question whether I know someone who can give me some guidance or not. And no, YouTube really doesn't replace practical experience. But maybe I'm just too much of a scaredy-cat ;)

We haven't had to do any handiwork for many years, except for painting a wall or something like that. We are cautiously approaching small things, but it's more like (you may laugh) "we have the first wooden outdoor table, how do you care for it actually/sand it down/paint it" and not the same for the fixed indoor stairs :S

With many tasks, you are more precise yourself (painting, laying floors, assembly) and it turns out better than from a professional. For some things like plastering or tiling without tools, you need an extreme amount of practice to achieve a good result.

But the quality in craftsmanship has massively declined due to overloaded order books and a shortage of skilled workers. It's rare to find someone who brings passion and skill.

That's exactly right. And there are a lot of things that are simple and only cost muscle power and maybe a bit of disposal. Especially in demolition. I wouldn’t install a roof window myself, wouldn’t build a dormer, generally wouldn’t go on the roof. Not proper electrical work, no built-in water pipes. Things like that. But there are many things you can really do well yourself and save a lot of money. You have to be willing to sacrifice time, of course. The work doesn’t do itself.

Sure. You should stay away from those kinds of things.
But installing an interior door yourself is really no rocket science. Someone showed me the first one (which turned out to be a bit crooked, I should have just done it myself from the start) and on Monday, alone with just wedges and 3 cut-to-length timber beams, I managed to install 3 doors... the only thing slowing me down was the fact that I only had 3 timber beams and the foam takes 1 to 2 hours until it really sets without pushing.

A professional probably charges a flat rate of about 50-80 EUR per door installation. "Earning" 150-240 EUR in one day...

Sure, if you earn 35 EUR net, you can probably afford the professional at 70 EUR/hour. But hardly anyone earns that much, especially not in the typical age of a builder, namely early to mid 30s.
I think I don’t earn badly and get a good 20 EUR net/hour. Plus allowances, expenses, travel times, etc.
That means for the 70 EUR that the floor layer charges, I work just under 3.5 hours.

I’d rather do it myself, in the last room I was already significantly faster than at the start.

It probably always depends on the person, the expertise, the expectation, and the skill. For me, it depends on what the work is for. For example, we are planning to tile our garage interior ourselves. My mother said she can help/guide, she has done something like this before. I never have, but in the garage it is not so important to me that it turns out perfect.

We also do quite a bit ourselves, and additionally have relatives who are skilled craftsmen. But there are things where I tell myself, it’s not called a vocational training for nothing, and you don’t become a master for nothing either…

We started masonry work on the upper floor today... Work on the ground floor began last Monday... so 6.5 days for one floor including concrete pouring and 1 day for electricians/ventilation to pull empty conduits. 2 masons from the company and 2 helpers from us – you don’t need to pay anyone €60/net for stone cutting or mixing adhesive, it works out in a similar time like this... I still get "homework" every evening from the boss which I can also do in the evenings as a non-construction worker... But I am currently at the construction site between 9 and 14 hours a day... But the saved amount is quite substantial!