Multimedia and Wi-Fi and connections

Conduits are available in every good hardware store in the electrical / cable protection section

Off the top of my head, I would say 25 mm or 16 mm.

As far as network cables are concerned, those of reasonable quality (Cat. 7) are already quite thick and stiff. In addition, duplex lines should be used (two "glued together" cables). Apart from that, I really don’t see that such cables will be outdated in five years. Even with "only" Category 6, you are still fully up to date today.

What you call a router is strictly speaking a combination device, namely (as a rule) a modem, a router, and an access point (Accesspoint). An access point is the device that *exclusively* establishes the WLAN connection or creates an infrastructure WLAN. A repeater is a signal booster. It takes the WLAN signal (from below) and retransmits it. Since range is a critical factor in wireless transmission and the connection quality (speed) drops rapidly with increasing distance, poorly connected areas can be hardly optimized this way. However, a repeater can only ever be a makeshift solution. It is better to have an (additional) access point, which in turn is connected via wired LAN.

By infrastructure outlets I meant the outlets that are not used for PCs or printers but "only" for network components like access points, routers, switches, ... These are sometimes located in different positions than the outlets for normally used devices.

Ah, okay. So I just connect the access point to the outlet via RJ45 and then I have Wi-Fi on the floor. Are there specific devices from FritzBox for that, or are they always combination devices? How can I connect a telephone to a network outlet - isn’t that a different connector?

That's exactly right. To my knowledge, there are no explicit APs from AVM. Theoretically, however, a repeater (with a LAN port) should be configurable in such a way - but I'm not sure. Other manufacturers offer something like that in abundance - usually even much cheaper than a complete Fritz!Box. As for the phones in RJ45 sockets, nowadays there are often RJ11 ("narrower" RJ45) cables included with an RJ11-TAE adapter. If you leave out the adapter, the RJ11 plug fits into the RJ45 socket. Otherwise, these cables are available for 1 EUR on eBay. If you have an ISDN phone, the plugs are the same anyway. And if you use the integrated DECT base station of a FB, you don't even need something like that.

Since I installed the network cables myself in my house, here is my experience with the empty conduits:

I ordered 25mm plastic pipes. I only managed to push the Duplex Cat.7 cables into the pipes with great effort before laying the pipes. I would never have managed to do it afterwards. In addition to the network cables already installed, I also laid a few empty conduits. Who knows what might come in the future.
In the same way, I also laid empty conduits for speaker sockets behind the couch.

With aerated concrete, I was able to cut the channels with a circular saw. If that had not been possible, I would have rented a slot cutter/wall chasing saw.

One more thing:
5-core cables from the motors for roller shutters and venetian blinds lead to the utility room. On long winter evenings, I can then solder together a control unit.
The power for the motors comes from the nearest power line and, of course, switches for manual operation are also recessed into the wall.