LAN and Wi-Fi "devices" per floor?

You don’t understand. I already wrote that you could connect a switch to the socket (with power losses). A double plug is not an option. A modern house has more connections in the office and living room than you have in total. That should answer your question. Wifi only for mobile devices. Everything else to the Lan.

We trusted the electrician with the electrical installation,

Yes, for the electrical installation, but with network planning this is usually not a good idea! Was there at least a measurement protocol for the installed cables? That would be the only reason for me to let the electrician do everything.

Since it sounds like you are not very deep into the topic, I would go for the Fritzbox. Where is the router going? The area around the router position needs good Wi-Fi. The choice of the model depends on this. You also do not need to put an access point in every room. Less is often more! What you really need you will have to test. If several LAN ports are required, then either a switch with an AVM repeater (but operated as an access point) or you could also use a Fritzbox from the 4 series and operate it as an IP client.

I don't find the situation great, but believe me, there are still new buildings without network cabling where you can only use catastrophe.

I'll join in here, we have about 14 LAN ports (CAT 6A, single) distributed throughout the house, which all converge centrally in a network cabinet in the basement. Therefore, a 19" rack 12U, a 16-port network switch, and an 8-way power strip were offered. The 16 ports seem tight to me with already 14 LAN ports. Assuming we decide on 3 more LAN ports during the corresponding construction phase, what would be the next larger switch size that we would need? Are duplex cables more useful than single cables?

Yes, yes.
Install duplex and double outlets right away. For the TV and office, feel free to install more.
Then go for a 24-port or 42-port switch. Also a larger patch panel.

With 14 connections, I wouldn’t take a 16-port switch but go straight for a 24-port switch. I also wouldn’t get the device from the electrician but organize it myself afterwards. Passive technology by the electrician, active technology done by myself.

Duplex cables are nothing other than two individual network cables that are "glued together." Which one to use, I would let the electrician decide. Always install each network outlet as a double outlet and in places where perhaps more devices are to be connected (office, TV, ...) also more than one double outlet. Then there’s the topic of access points for the Wi-Fi. Personally, I would only put these on the ceiling and operate them via POE. Of course, a cable must go there and a small box to hide everything behind the access point. If the ceiling is not possible for some reason, then up high on the wall. Positioning of the access points depends on the floor plan and user behavior. You can’t say anything without information on that, except that usually at least one access point per floor is installed. Usually only the living space is considered in the network planning, but you should not forget the rest either. Outdoor access point, network in the garage (wallbox), network for inverters, network for the intercom system, cameras, ... Here we are back to POE, and when you have many devices, you take a POE switch. So you see that only with the existing components can you say what kind of switch you want or need.

If you use cheap APs, you have an old standard. Therefore, take a Fritz!Box and FritzRepeater2400. Set these up as APs. You will also have a LAN port on the access point.

ok.
Do you then consider the FritzRepeater 2400 as a somewhat new standard?
and with FritzRepeater then waive POE (?!), because...(?)...