You may be referring to Unreal in this instance. Unreal is open source* but you pay to license the engine, so you can make core changes I.e. memory management or tweak underlying net code. Unity is closed source and so you can only really extend very core parts of the engine.

FWIW my experience in gaming has been that Unity is exceptionally powerful and allows game developers to create games that would otherwise require an entirely separate dev team to support the engine. When I first learned game dev, the code sections were almost entirely devoted to interacting with the underlying graphics libraries (OpenGL or DirectX) and hardly any to creating powerful features in a game. Now, using something like Unreal or Unity is akin to using a web framework like Ruby on Rails or Laravel.

* The source is available to view and modify