Subsequent enlistment without the general contractor

But does every inspection in the house have to be done in consultation with the general contractor (GU)?

If it’s stated like that in the contract, of course. They also provide you with the warranty and guarantee the date.

Until now, I thought you could arrange this at the appointment with the electrician on site, when you are going through the shell of the building anyway and decide where the individual sockets should be placed

That’s possible. But the GU might also veto it.

Now the question, is it really possible that such contractual relationships exist between the trades and the GU? How do I find out about this without stepping on the GU’s toes.

Look at your contract for work to see if you also have authority to issue instructions to the trades. Usually not, that’s why it’s a GU. Annoying small stuff can and often is handled directly between the craftsman and the client, so the socket, yes. The electric roller shutter or the electric garage door, no. Often especially with low-budget GUs, the trades work roughly at cost price and then make their profit with hefty surcharges. This works particularly well when the standard is rather low so that practically all clients will upgrade. For example, in our case, 25 sockets per house were standard plus 1 network socket. Each power socket 60 extra (i.e. a triple row for 180 bucks), network socket individually 170 euros and 220 euros double... I already knew this beforehand and simply incorporated the desired equipment into the purchase price. It fit! By the way, I find electric roller shutters for 300 euros okay. Otherwise, you can negotiate a package before signing the contract, for example about 10 duplex network sockets for 700 euros. Afterwards, it’s "take it or leave it"; you really don’t have alternatives since doing it yourself is only possible after acceptance and then you hardly want to chase fresh plaster anymore...

Really expensive

is relative. I put up 170 bucks in return...

We also had everything related to electrical work that came afterward directly inspected by the electrician...

As an example:
LAN/SAT outlet 40€ each
Socket 13€

The same in the bathroom. We also had everything directly inspected by the plumber there. And he really gave us fair prices.

There were actually no major quantity increases with us. Everything was discussed and priced in advance by the general contractor (GU). Something additional came up with the electronics, which was handled directly with the craftsman. We wanted sockets with integrated enhanced touch protection, which the GU had forgotten; this was handled directly by the electrician. The special stair painting, on the other hand, was done through the GU. We only realized this shortly before installation at the stair studio. The surcharge was reasonable.