It's linked into Mach-O binary that is loaded by iBoot bootloader. It is explained in detail there: https://www.youtube.com/watch?v=d5s9fYfvzmY&t=12260

I don't know how it works on apple hardware—though I heard they used a UEFI variant with HFS+ at one point—but in general, on PCs, there are two ways to get bootcode to run.

The first is legacy BIOS: the BIOS will load the first 512 bytes of the boot sector, called the Master Boot Record (MBR) into memory location 0x7C00; they are then responsible for bootstrapping the rest of the system. Usually by doing the bare minimum to load a second-stage bootloader off the disc, and letting the second-stage bootloader do the rest of the work, since 512 bytes (actually only ~440) is really not very much.

The second is UEFI. The firmware will load a PE-formatted executable from a .efi-suffixed file on a FAT32 partition which has the EFI System Partition (ESP) flag set. Since this PE file can be any size, a second-stage bootloader is not generally necessary (but may be interesting for aesthetic reasons e.g. if you don't like PE).

On Rockchip boards, there is a bootrom that looks for data at a specific offset on a few storage devices, then calls the first one it finds. From there open source / flashable firmware initializes RAM, scales up CPU frequencies, and starts looking for a kernel or whatever.

TL;DR likely read only firmware, possibly more low level than a BIOS, UEFI, or Coreboot implementation would be.

Edit: then again, they could be shipping a full EFI implementation like they do on their x86 offerings.