The Open Source Society University is basically a GitHub repo which has all the components of a traditional CS major with resources in one place. Does somebody know of something similar for a mathematics degree?
Not including all the resources, but in my experience many (German) universities tend to have very comprehensive examination rules which list all offered courses with brief rough lists of topics covered.
For these the standard references are various textbooks, though almost none are ebooks and even fewer are freely available (scripts sometimes are).
The hard part about learning math is not finding course material. Three or four textbooks can cover the entirety of undergraduate mathematics. The real challenge is in the discipline, focus, practice, and repetition of doing problems over and over and over again. It can be almost impossible to force yourself through this, and it's really easy to start glossing over things you think you understand. The only real way to learn math (short of possessing genius level autodidacism) is in a classroom setting, with a teacher and peers, receiving constant feedback and assessment. I would strongly suggest you check out the local university or community college if you have a serious desire to learn.
I don't know of any degree granting programs but if you want math training all the way through a college level try Kahn Academy at https://www.khanacademy.org/#/math. Hope that helps a bit.
Most universities have their syllabus freely available on their websites. For example here is the one from Cambridge University:
https://www.dpmms.cam.ac.uk/study/
