How do I get WiFi and phone service, technology in the basement

Do you perhaps mean access point?

I interpret this

Because of the underfloor heating, I planned a CAT7 connection on every floor.

by the way also in such a way that due to the shielding from the underfloor heating, a WLAN (especially from the basement) would hardly have a chance to radiate significantly beyond the ground floor. First of all, a good decision!

I would not really advise against a Fritz!Box for the average user, who by their own admission is not IT-savvy. Not everyone needs IT gadgets, and if it just has to work, creating a wired mesh WLAN also works well and quite easily with Fritz!Boxes or Fritz!Repeaters (which are then used as access points or WLAN bridges). That it might offer more control and possibilities with other hardware is another matter, but that also requires more knowledge.

Last but not least: DECT/phones can also be easily managed over multiple Fritz!Boxes as bases. Advantage: DECT range tends to be greater than WLAN, especially with 5 GHz, so you do not have to switch from one cell to another as quickly when moving around the house. Otherwise, for example, the Fritz!Fon app also works, which then emulates a landline phone on a smartphone via WLAN. However, this would probably only work well on a single floor. Once you change floors, dropouts are to be expected.

I wouldn’t really advise against a Fritz!Box for the average user who says they’re not IT-savvy.

For my part, I am absolutely not IT-savvy, but I think that a unifi access point looks worlds better on the ceiling than some other box. I hope I still manage to get it running.

yeah, sure Accesspoint. No idea why I always write AC. I'm usually not a fan of abbreviations. "Accesspoint" is just so annoying to type xD

By the way, I see it the same way, within "I just want Wi-Fi" and for example "plaything/tinkering urge" AVM is already very good. A guest network also works without any problems.

which will inevitably lead to problems with clients because they do not know which of the two they should connect to best.

Just read that. Of course, that is nonsense. A client will usually prefer the stronger access point with the same SSID. It is important that both/all APs or SSIDs use the same security settings. It does not matter on which frequency or in which frequency band the APs operate. There is something like band steering, but in my experience, it works rather semi-well and I would initially leave it out of consideration.

No idea why I always write AC.

There is actually AC (Access Concentrator) as an abbreviation, but I have never read it in connection with WLANs.

Just read it. That is of course nonsense.

That is absolutely not nonsense. Just say that to devices with bad WLAN chips like a Fire Tablet or every second mid-range phone. They do endpoint hopping without end because the signal from one access point does not always reach the device well, and the device then switches.