Unity is definitely a big player on all platforms, but it's probably strongest on smartphones / AR / simulation for AI / and indie developers. Epic's niche with Unreal is more high end AAA games / big studio console releases, and now apparently Hollywood & VFX. Disney and ILM are using Unreal for real-time on-set backdrops in shows like The Mandalorian:
https://venturebeat.com/2020/02/20/ilm-reveals-how-it-used-u...https://en.wikipedia.org/wiki/List_of_Unreal_Engine_games#Un...
The underlying C++ source code is available for both commercial engines, but Unity charges high fees for source access, and only on a direct per-studio basis, whereas Unreal 4's source is available on GitHub if you pay $20/mo. The vast majority of Unity developers work within its IDE and C# API. There are definitely strong network effects from the sheer number of developers using Unity, such as the amount of documentation, tutorials, and C# code available online.
But again, Godot is fully open source and getting consistently better as it evolves. It's C++ based, closing in on competitive rendering, ported to every relevant architecture, and has a full IDE and scripting system. It reminds me a lot of the Blender project. At some point, some significant video game IP will be built using it and shake things up. It's just a matter of time.
