Rod Smith and the OSDev Wiki went here, sometimes word for word, years before:

* https://rodsbooks.com/efi-programming/

* https://wiki.osdev.org/GNU-EFI

Of course, they both assume GCC on Linux. Change either one of those, and it is a wholly different kettle of fish. Both NetBSD and FreeBSD, for examples, come with an EFI development environment in base, squirreled away in /usr/src/boot in the case of FreeBSD. They have to, because they need to be able to build their own EFI boot loaders. NetBSD targets more than Intel processor architectures for EFI, moreover. (NetBSD can boot via EFI on ARM CPUs, for instance.)