I'm running an S20+ now. Lots of 5G up in my area. There are a few places where I get a fallback to 4G service. The latest firmware update does a hard switch between my home WiFi and 5G. Early releases allowed both to work concurrently. It saves battery to be on the WiFi as I work in my basement office. The CDMA has to crank up the power to overcome my location.
I actually considered keeping my iPhone X from 2017 but I wanted full 5G access. Which is great because I’ve seen as high as 850 mbps download speeds on my iPhone 12 on the T-Mobile network.