You don't need to be able to build them. Just sign them or sign the keys that sign the third party blobs/binaries. Then your motherboard firmware will be able

You don't need to be able to build them. Just sign them or sign the keys that sign the third party blobs/binaries. Then your motherboard firmware will be able to load your GPU and other third party blobs to UEFI memory. Similarly OSes like Linux and Windows enforce the same chain of trust (they don't have to but otherwise it is not really secure, just like a website can lie to you about encrypted storage) so you need your drivers/OS loaded firmware to be signed as well. What Android does and what UEFI does are not really related. It is like comparing how SSH does authentication vs how HTTP with TLS does. Former is a SSH-specific open-ended implementation detail, latter is standardized by IETF. Similarly UEFI standardizes how a motherboard manufacturer can write a compatible firmware and Secure Boot (capital letters) is a sub specification of UEFI. It is not the only secure boot implementation scheme. With Android device manufacturers have complete control over the early boot firmware and the OS. As long as they boot the OS to run apps, how they do it is up to them. Only things like Google's SafetyNet will put certain requirements on them. No standard like UEFI exists in Android phone world or anywhere else except PCs / Servers.