Dear all #Unity developers scrambling to see how to even begin to port your games to another platform: you're welcome. https://github.com/barcoderdev/unitypackage_godot
In case you're suddenly facing the task of porting an entire game over due to capitalist nonsense or something
I was able to port over several of my demo projects quickly because I originally wrote C# console applications and then refactored as I went when bringing it into a Unity project. I find this approach is actually easier for structure, as it decouples my code from the engine except for places where it's absolutely necessary. I didn't build it like this with the intention of moving over, but it made more sense to write my own logic for my own objects rather than make a game with a shit load of the bloated GameObjects Unity offers.
Swapping over to Godot was a breeze, all I had to do was learn the Godot specific calls for mostly the same stuff and plug and play. Super easy
True, it really depends on how the code is structured, if the project from the beginning have been written to support multiple engines. Or more likely totally integrated with a single engine.