And the user interface is all OpenGL... man I can't imagine how much time it would take to write a decent user interface starting with nothing but OpenGL. Like just rendering text to the screen is a pain. And it's so snappy and responsive and it looks sooooo good. Definitely not a one man project, there's just no way.
I recently started doing basic modeling work for my small hobby projects. I'm amazed at how professional and finished Blender is for being an open source project. It feels like I'm using an Adobe product or similar. Big props to the team behind it, you're awesome.