rlgl is an abstraction layer for various OpenGL versions (1.1, 2.1, 3.3 and ES2.0), depending on the selected version when compiling rlgl (using flag GRAPHICS_API_OPENGL_xx
), rl*() calls are adapted to different OpenGL versions.
raylib is not primary intended to learn OpenGL, it actually tries to make things simple to avoid worrying about OpenGL... but you can check rlgl code to see how OpenGL is used, code is quite organized and commented.
There is also a rlgl_standalone
example to use only that abstraction layer independently of raylib.