The trick is that since Unity itself is not permissively redistributable, the ability for the game to work depends on both Unity the corporation continuing to p

The trick is that since Unity itself is not permissively redistributable, the ability for the game to work depends on both Unity the corporation continuing to provide its proprietary part and to make it available under terms allowing games to be compiled with it. I would consider neither Unity nor Unreal capable of producing free software titles because there is no way to get the engine parts without some barrier. It may not be monetary, but you still need to agree to EULAs for both engines, despite the later being nicer since it also includes source access. An open source game does not, however, need to start from scratch (which saying OpenGL seems to imply). There are both legacy code bases like doom3 or free engines like Godot that let you build off them rather than reinventing all of the extremely complex and sizable code required to even start making a video game. It is just you cannot have open code that is then tied to a proprietary part unless that proprietary bit has liberal distribution rights (and I don't mean just the game lib you ship, you need to be able to recompile the project with changes which almost always means you need the game editor). I'm not aware of any proprietary engines that give away all the tooling under permissive terms that would fit that scenario, though.