A Software Defined Radio is close to how a cell phone functions. Most of the functionality for camp-on, tuning, frequency hopping, diversity receiver sensitivity, transmit power are handled by hardwired algorithms embedded in silicon memory of the modem. The complexities associated with 4G radios are large enough. 5G (or LTE - Long Term Evolution) is a bigger hike in complexity still.
YES. There are massive trade-offs associated with small cellular phone antennas. A pentaband antenna (such an antenna covers most of the cellular bands) rarely surpasses an SWR of more the 3:1. 1:1 of course is ideal. And, as 1 respondent to your question has mentioned, I too regard the RF boys as effectively magicians in respect of how they manage to make an antenna radiate effectively at all while surrounded by conductive clobber carrying harmonically related signals of bewildering variety. It takes the ‘art’ of tuning an antenna to a whole new level. And yes, some phones even modify the resonant properties of an antenna on the fly to obtain a sufficiently usable VSWR.
Hope this does not confuse further.