> they can dynamically allocate subchannels in each mode based on the immediate needs of their users.

Right, so if I'm still rocking 4G only while 'everyone' else in the area is using 6G with some other stragglers on 5G, ...

They're (mostly) the same channels shared by all of them. It's not like an ethernet address (where everyone on the network speaks the same protocol and just ignores packets not for it), it's a radio frequency timeslice that's allocated to you, and you and your nearby tower can speak any radio modulation they want in that timeslice, even ones that other phones or other towers don't support.

However new technologies sometimes also bring new spectrum blocks like mmwave 5G.