Commissioning of the network installation

Because then all network devices are actually centrally located.

But the more I read into the topic, the more I also assume to set up the FB in the living room.
Then secure the signals upstairs using an access point.

If the FB is in the living room, one network double socket should actually be enough, right? The spots on the wall are then occupied, but I still have three free ones on the FB. I only use the TV, very rarely the Blu-ray player, and maybe later a LAN-capable AVR.
WiiU and FireTV receive via WLAN.

If the FB is in the living room, one double network socket should be enough, right?

Only if you only route Telekom through one socket and Internet through the other. But what if you still have a fax or an ISDN phone somewhere that you want to connect to the FB? Then you need corresponding additional ports, unless you fiddle with the cables (not all wires are used). Ours stays in the utility room, there will probably be a separate access point in the ground floor hallway, so far it is still a repeater.

Because then indeed all network devices are centrally located.

That is an end in itself, but of course you can do it that way! For me, I see more advantages in placing the Fritzbox in the hallway: Without any cable tinkering, I can also centrally place the base of the phone next to the Fritzbox, and at least on the ground floor I have DECT and, of course, centrally located Wi-Fi.

I believe, however, that the idea is to completely do without a base, or to use the [FB] as a base.

Such a device is placed centrally in order to remove the cable clutter from the living area. In addition, as already mentioned, a fax can easily be patched from a central location to a data socket and the Fritzbox can be patched to the LAN.

That is the problem with devices that have all functions integrated. I would actually need the modem/router in the basement, where the patch panel is, but WLAN and DECT not there, rather on the ground floor.

I see two possibilities: 1) Fritzbox in the basement, DECT registers directly there, disable WLAN and use APs on each floor. 2) Fritzbox on the ground floor, DSL comes from the basement via the telephone socket, LAN is routed back to the basement via cable. Additional access point on the first floor.

How do you want to manage the handover between the APs for WLAN? I.e., when going up the stairs, being seamlessly handed over to another access point? Simply a separate access point with its own SSID or have you looked into roaming?