What is best for filling holes in masonry? With construction foam?

Unfortunately, we also had to experience this, which leads to us doing more and more ourselves

Blower door test leakage in the electrical installation: screed is scheduled, the electrician certainly won't come by for such a small thing, the screed guy doesn't care, he just covers it up with leakage, so who fixes the whole thing quickly now?

100 points. The builder.

Same with the outlet in the ground floor ceiling for the underfloor heating: plumber says he won't do anything, the structural builder won't come to fill such a small hole. And before someone just fills it with foam again, I'll do it myself.

This happened to us quite often as well. It's obvious when you award all trades separately or do them yourself. The interfaces are often not so easy to define. But I much preferred being able to do such things myself. If someone else had done it, it would have just been done quickly (botched) again. Therefore, I don't even want to know how often such botched jobs are done by a general contractor or similar.