Yes, the hardware is peculiar in many ways and Apple didn't bother to document it (it would have costed them some resources for no clear benefit to their business model) but they also didn't bother to completely lock it down, like they did with the iPhone and iPad. This is what allows other OSes to be ported to it if they are able/willing to reverse engineer and develop support for it.
This reverse engineering work was done so far by just a handful of people who would also like to have Linux run on it, partly sponsored by a few hundreds that share this desire, or just doing it voluntarily as a challenge/interesting project.
The m1n1 bootloader/hypervisor built by the team allows running macOS and logging all the hardware access.
Once the reverse engineering is done for a given component, all the other OSes can implement support for it.
The fact that a lot of the functionality is implemented in firmware blobs makes it easier to support at the OS level, essentially each OS driver only needs to give high level instructions to the firmware, without having to implement all the minutiae done in the firmware.