I don't see anyone mentioned
https://ocw.mit.edu/index.htm
which covers all topics.
There's a number theory course on YouTube
https://www.youtube.com/watch?v=19SW3P_PRHQ
based around a Dover book. I have not watched much of the video. As the author points out, Dover books are quite reasonably priced. I have bought the book, and intend watching the video, using that in conjunction with the book.
The Open University put a bit of a course on number theory on their website.
https://www.open.edu/openlearn/scien...tion-0?intro=1
However, I think the amount is pretty small compared to other resources mentioned where people have put whole books, or several chapters of books freely available. As such, the OU course might not be worth bothering with.
Dave (engineering graduate).