I used to work with 4G, so I could probably answer some of your questions, especially about the signaling/coding/etc, but not so much protocols.

Still, your question is very broad and vague, and I've got to admit I've got a very hard time thinking of stuff I could tell you which you would be able to understand, but which would still be interesting. The fact is that 4G, and for that matter 3G or 2G, are complex frameworks which entail much more than a single concept which can be simplified for the layman.

AM or FM radio are simple concepts, due to being from an era when everyone didn't have a bunch of microprocessors in their pockets. There was pretty much no concept of "state", except for tuned frequency. When people speak at the same time on the same frequency, they speak over one another, like two people in the same room.

With digital communications, like 2G and later, your phone and the base stations (the antennas which receive your signal, and send you your input signals) no longer simply send a signal on a single frequency, representing the audio from your voice. Thanks to digital processors, the signals can be coded and decoded in such a way that they are sent over a small range of frequencies, and time slots. Something like this (this is actually illustrating 4G, but the concept is similar for 2G):

In 4G, any user can receive data on any one of these squares, they don't have to be in only one frequency. The base station tells you which blocks are for you. Because of this, several users can be on the same base sation, using the same frequency band, and still not hear each other. Your phone takes the blocks which are assigned to it, and reconstructs the caller's voice from them.

In 4G, everything is simply data, from text messages to voice calls. There are no special modes for voice, as I think there is for 2G and 3G (again, I'm no expert on these). Every one of the blocks contain a signal which represents some binary data (1s and 0s), which in turn represents anything from a website to a voice call. The protocols tell you what the different blocks mean.

There is more to know. Much, much more. However, I don't know where to start, or if you even want to know more. I hope this post isn't too vague or confusing, but it's legitimately difficult to explain this. Does this answer your question?