As an aside, the IMS stack used to implement SMS/MMS/RCS on Android is super cursed. A lot of the heavy lifting is handed off to the OEM, for example, Pixel devices hand it off to the Qualcomm modem. (Meaning Android the OS doesn't even have any control over how the raw SIP messages are sent: they're inside an IPSec tunnel set up by the modem that it can't see inside)

iirc Samsung devices do it differently and they implement it in userspace using StrongSwan?

That's why it's super annoying to handle SMS/MMS using the standard/legacy APIs, because depending on what device the user has, the implementation may behave radically differently with regards to PDU parsing and such.

RCS makes the whole situation worse because it sets up an entire secondary IMS stack inside the Google Messages app, and then uses weird APIs to try to tie it back into the main stack, even though obviously the modem implementation doesn't understand RCS... it's a mess.