In France ethernet with RJ45 is the standard that replaced the older phone line that we were running in the walls. Since you can plug RJ11 to RJ45, you can use a

Any decent electrician is France knows how to install ethernet, and they can get install this kind of equipment, equivalent of a distribution cabinet, instead of the regular "phone line" box:

https://www.tableau-de-communication.fr/coffret-hager/13-cof...

https://www.tableau-de-communication.fr/coffret-hager/14-cof...

When I had work done in my house I just made sure that they knew that before they boarded up a particular ceiling I needed to run a cable. In the end we all ran the cable together, and now I have a 1Gb/s Ethernet connection to my office.

My house built 16 years ago has 2 4-pair cables of CAT-5e run to every room for phone wiring, standard from the home builder.

The blank stare was that I asked for conduit and pull-string everywhere, and they weren't willing to do that. Or to do any variants beyond the standard 2 cables to each place, so my office has 4 ethernet ports (adding another "2 cable jack" was something that they could do) and needs its own switching.

Depends where you live. Here in Canada almost all new builds have cat6 run to each room to a central closet.

Problem now is that most electricians think that because "it is just low voltage wiring" you do not need to worry about interference and can just run it anywhere.

maybe try a different builder then.

those are cables and sockets, standardized to the point of beeing a commodity. no magic involved at all.