The key aspect seems to be that there's a commercial entity behind the project who presumably has console development licenses.

AFAICT none of the console specific source is Open Source but the implication seems to be that the source is (freely?) made available to registered console developers due to the fact the developer of both is a licensed console developer and has decided to make it available to others.

Console development/distribution is mentioned in a couple of places in the documentation[0]:

"The hello.hl file contains bytecode that can be run with the HashLink virtual machine using hl hello.hl. It does give quite good performances and have been proven by successful commercial games such as Northgard or Dead Cells.

However, it is also possible to compile the HashLink code using a native compiler. This allows to compile for consoles and mobile.

This will create a directory out containing a lot of generated C code that needs to be built using a native compiler and linked to the same HashLink runtime[...]"

With regard to console support it says:

"... for Consoles (Nintendo Switch, Sony PS4, Microsoft XBoxOne), please contact us at nicolas @ haxe.org if you are a registered developer for one or several of these"

And here[1]:

"The final resulting shader will then be compiled to the target platform native shader language (HXSL currently supports GLSL for OpenGL, HLSL for DirectX 11+, AGAL for AdobeAir, and PSSL for Sony PS4)."

Also, this post[2] describes how the lower level parts (using Haxe/hashlink) integrate with native (i.e. console) platforms:

"The game gets compiled to a cross-platform .hl file that can be run with HashLinkVM JIT. It can also be compiled to C directly and compiled using any native compiler, which we are using for our console ports on PlayStation, Xbox and Nintendo Switch."

"We also have additional platform API libraries for Steam, as well as console integration libraries that are only accessible if you have a registered developer."

In the same post the Heaps.io console implementation is described:

"This architecture allows us to integrate new renderers or platforms by just porting a few classes given that the native libraries are been made available in HashLink. Heaps.io supports the following plaforms/renderer:

[...]

HashLink/C with NVN (Nintendo Switch SDK native graphics API)

HashLink/C with GNM (PS4 SDK native graphics API)

HashLink/C for Xbox One SDK"

[0] https://heaps.io/documentation/hello-hashlink.html

[1] https://heaps.io/documentation/hxsl.html

[2] https://haxe.org/blog/shirogames-stack/

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!)