What is the name of android's equilivent to UEFI?
The firmware is more related to hardware (and hardware type/architecture) than to software/OS. OSes must support that hardware/firmware but the firmware is not part nor exclusive of the OSes.

ARM is the typical architecture where Android runs and yes, it varies a lot by manufacturer.

Replicant OS is geared towards ARM based hardware and, according to their FAQ, is currently supported in few devices and some of them only partially, I imagine because getting support for a specific device depends on how "open" the manufacturer is about it. And there are limitations because there are lots of proprietary stuff that's needed for full support, there are features missing without non-free firmware. Definitely not as easy as your typical PC.

However, a discussion about the specific devices you're interested in - Samsung Galaxy phones - is off-topic here - https://superuser.com/help/on-topic - but it's probably a good question at https://android.stackexchange.com/ .