> Pixel devices hand it off to the Qualcomm modem.

Older ones maybe, newer ones use Samsung Shannon modems.

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

Just to be cursed the entire IMS stack for Samsung on both qcom and exynos is custom. It’s why no custom roms have support for voLTE on their devices. They also had their own RCS that they sunset in favor of Google Messages.

Apple’s implementation is cursed too and not only did they use an older specification but they didn’t fully implement required responses to registration issues, I wrote about it here: https://wt.gd/working-rcs-messaging