Godot is an amazing 2D engine, and with the upcoming release of 4.0 it will
be just as good for 3D too!
It is entirely free, and is easy to learn because it uses the languages
C# and GDscript and it's own visualscripting language.

I would recommend it as an alternative to unity/unreal, or
just as a c# graphics library as it is incredibly quick to setup, (it works 
in most modern browsers, or as a desktop download, with no install needed!