As pointed out in the comments, 6.033 is a good overarching course to learn about a lot of computer systems fundamentals. If you want to dig deeper into networks, OS, computer architecture, MIT has plenty of specific courses for those:

http://web.mit.edu/6.829/www/currentsemester/ https://ocw.mit.edu/courses/electrical-engineering-and-compu... https://ocw.mit.edu/courses/electrical-engineering-and-compu...