There are some open firmware, or partially open firmware projects, but they're more proof-of-concepts and not popular/widely-used. The problem is the FCC or corresponding local organization requires cell phones get regulatory approval, and open firmware (where just anybody could just download the source and modify a couple of numbers to violate regulations) doesn't jive with that.

https://hackaday.com/2022/07/12/open-firmware-for-pinephone-...