You generally pick one interface and do everything on that. The modules are still using old school AT commands (so you still send ATD to dial a phone number). Yeah the 4G uses the same AT interface over serial.

You picked up the drawback: speed. You can't pump 4G speeds over a serial port. Depending on what you're doing through, the 'slow' speeds you get over a serial port are more than enough to serve your purpose. The 4G part is about connectivity and not speed in many cases. Locally we're seeing 3G shutdown and it won't be around for much longer.

Obviously better solutions exist for low data usage over cellular - have a look at NB-IoT or LTE-M.

With modern 'system on a chip' modems, data goes from baseband to application silicon over shared memory via a proprietary interface:

https://en.wikipedia.org/wiki/Qualcomm_MSM_Interface

I think non SoC high performance modems use USB.

Regarding older or less integrated modems that do use the 232 interface, this may be illuminating:

https://fahrplan.events.ccc.de/congress/2011/Fahrplan/attach...

yeah this looks correct and it's a different world if you have direct access to the modem IC.

To clarify I was referring to the modules that the average person can get their hands on. If you rip open the module (or look at the FCC paperwork) you'll likely see a Qualcomm chipset underneath. It's just a level of abstraction really.