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.