If you want classes that are more advanced or that go into greater depth, I recommend the courses offered by edX or Stanford-online or MIT OpenCourseware. These are full-term for-credit courses with video lectures at top schools that you can 'audit' for free, though few or none will grade your homework or projects unless you pay full tuition. By contrast, 95% of Coursera or Udemy courses are much shorter and more introductory.