[ndrarmst]

The counterargument to an all-software solution is that if you switch the profile on the Fi phone, it also remains switched if you move the SIM to another device. So however selection was made, it is 'sticky' across devices.

That's why I guessed it was a paired SIM app + intelligent Android app - but I couldn't verify that, so you could be correct.

[a-dub]

Usually radio stuff happens on a dedicated processor which has limited communication with the application processor that runs Linux/Android/Java/Userland. (think like a modem, in some cases they are literally controlled with serial links and AT like text commands).

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

I believe that profile swaps would be sticky, but actual processing on the SIM is going to be extremely limited and active management of the radios most likely will take place in baseband firmware.

Android talks its baseband processors by interfacing with a daemon called rild (Radio Interface Layer (Library?) daemon) which loads a binary blob library which actually does the talking to the baseband chip.