What does a "classic" electrical installation look like?

Yes, your picture shows both light and socket on one cable with a junction box. That is no longer done. Separate light and sockets and without a junction box.

Conduits cost little and it is better to take a few more. Besides the fuse circuits and separating lighting and sockets, also consider telecommunications and data cables: these are, on the one hand, preferably not stuffed into the same channel as the lighting power and, on the other hand, installed without branches, daisy-chaining, and the like. Nowadays, neither the "socket system" with daisy-chained TAE sockets is done anymore*, nor should one overestimate and think that cables are no longer needed thanks to WLAN. And a double socket is also fed by two cables (each nonstop from the connection room to there). I would currently even consider having fiber optic cables alongside the copper cables in a few years.

*) especially regarding IT & TK, as a builder you have to become knowledgeable yourself, because very many electricians are not yet: they still wire in the old network topology and think that by using Cat.6 or Cat.7 cables instead of the old red star quads they have already installed the future

Yes, but your picture shows light and socket on one cable and with a junction box. That is no longer done. Separate light and sockets and without a junction box.

Cable very well one per room, but with multiple phases. So that light and sockets are separately protected.

I am currently working on it as well. Coincidentally, the marking starts tomorrow, and over the weekend we will install the first boxes. Since I am more active in the industry, I have been dealing with this more intensively over the past few weeks. Here are a few tips that might help you when you talk to your electrician. Basically, he should install 5-core cable and not 3-core. Technically, both work, but with the 5-core you have the option for 2 additional circuits later. And the price difference should not be more than a few €. Bus systems can also be easily retrofitted later via the cable. Some people will now complain because these are not data cables, blah blah blah. Technically, it works perfectly. Roller shutters. Lay the cables here accordingly so that an upgrade for automatic operation is possible later. Nowadays, many providers have corresponding radio transmitters with which you could create scenes or similar. Sockets. If you are planning a socket in any corner anyway, just add one more for a double socket. The effort for a double socket is no more than 5 minutes longer than for a single socket. Simply install a second box and bridge both boxes internally. Data cables belong in empty conduits. If they just lie in the screed and get damaged, you will never be able to use them again. And pulling new cables throughout the entire house is too much effort. Think of the outdoor area. The cable only costs a few euros and you are building anyway. Just have one laid on each side and additionally a cable with a larger cross-section for an electric car later. Who knows where the journey will go.

I have another new question. Is it possible, instead of 2x cables NYM-J 3x1.5 mm² for 2 circuits, to lay 1 cable NYM-J 5x2.5 mm² into the room and then split it into 2x NYM-J 3x1.5 mm² for the sockets and lights? And how does that affect the cable length? Is the 17m at 1.5 mm² calculated from the distribution box or from the split at the 2.5 mm²? I should then be able to split the 2.5 mm² at the distribution box to 2 fuses, right? That way I would have sockets and lights separated, correct?

I have another new question. Is it possible, instead of 2x NYM-J cables 3x1.5 mm² for 2 circuits, to run 1 NYM-J cable 5x2.5 mm² into the room and then split this into 2x NYM-J 3x1.5 mm² for the sockets and lights? And how does it look then with the cable length? Is the 17m at 1.5 mm² calculated from the distribution box or from the split starting from the 2.5 mm²?

e. I should also be able to split the 2.5 mm² at the distribution box onto 2 fuses, right? So I would then have sockets & lighting separated, correct?

Technically it works. At least partially. But that’s not how it’s done. You would necessarily have 2 circuits in some junction box. If those are on different phases, then it’s close to a short circuit. If it’s about a bedroom, you can simply use 5x1.5 and have one circuit for sockets + lights. In the living/dining area, I would use 2 cables each 5x1.5, one for sockets and one for lights.