Sadly, most of the big names (Udemy, Coursera, ...) are using encrypted streaming with DRM, but there are Khan Academy, Harvard, MIT,... courses that you can do

Sadly, most of the big names (Udemy, Coursera, ...) are using encrypted streaming with DRM, but there are Khan Academy, Harvard, MIT,... courses that you can download and use locally. But most people are downloading the courses from torrents illegally (I don't advise doing this).