Essentially the main parts are probably:

* Support the platform native C compiler. (e.g. convert your build system & any idiosyncrasies in your/their code).

* Support the platform OS.

* Support the platform native graphics API.

And, yeah, the easiest way to do this (while still maintaining cross-platform compatibility) is to have an abstraction layer of some sort between the game engine & console specifics.

And, Godot does already have this abstraction layer to support multiple platforms, it's just that everyone who wants to use it on consoles has to reimplement it themselves--unless they work with a third party who has already ported it.

(AFAICT there's nothing that would stop a company from doing a console Godot port and then saying "We can't Open Source this code but will license it freely to anyone who can prove they're a registered console developer" but I'm not sure what the commercial incentives for that might be.

It'd reduce unnecessary duplication and could in theory allow for (licensed developers) to improve the code base in cooperation but would be quite the undertaking & also a significant but also necessarily invisible community contribution!)