What is the name of android's equilivent to UEFI? Sorry if this is a dumb question. I'm having the hardest time finding the name of android's analogue of the Unified Extensible Firmware Interface.

Maybe they are specific to each manufacturer? I am specifically looking for the one used with Samsung galaxy phones.

Is there an opensource specification for interfacing with android phone hardware, similar to how UEFI and BIOS works?

The closest thing I have found is Replicant OS. The description says "Replicant is a fully free Android distribution running on several devices". If it can run on many devices would that mean its using the same firmware 'interface' across many devices?
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/ .