Boot firmware (called BIOS on x86) is much different on other systems. Commonly used ARM SoCs have bootrom (real ROM in the chip: just like circuit) that boots a bootloader from NAND (the phone memory where the OS is usually stored) which might be signed like in restricted boot. (There might be more stages and the names are usually different: I don't know the details.)
Most Replicant-supported phones have a nonfree bootloader in NAND which is signed (not replaceable on nearly all phones except for flashing nonfree updates from the vendor) with the exception of GTA04 using free U-Boot.
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