Most operators do cellular over wifi now (because 4G/5G sucks indoor). Not all of them though.
EDIT: I searched harder and found “VoWiFi”. It looks like this can support SMS and is supported by iOS. TIL.
Essentially VoWifi connects to your ISP via a IPSec VPN using secrets derived using your sim card as a smart card, then connects to a internal SIP server and voila.
I spent a lot of time trying to connect to the IPSec server from a computer, but couldn't figure out how to get the challenge solved by my sim. Gosh I wish I could make normal calls from my computer.
But 5G is mainly done on 3.5Ghz and that's tough yes. Of course they can use the same frequencies for 5G. Though the bandwidth at such low frequencies is limited, and thus the maximum speeds.