If what you need is lectures, you'd download them from MIT OCW or watch them on YouTube or Khan Academy. Or possibly just read the textbook, which may also be free online, or you can download it from Library Genesis (if that's legal in your country), or buy it used for US$35. Also, I can strongly recommend Wikipedia and... visualization videos that aren't exactly lectures, like 3Blue1Brown stuff.
You wouldn't pay tens of thousands of dollars in tuition for lectures unless you were the kind of person that universities like Columbia try very hard to not admit.
Classes, however, are very valuable. You can ask questions when you don't understand things, you can talk with the other students to build shared understanding, you can get your homework graded and confirm that the things you weren't sure about on it were actually correct, you can take exams and test your skills. And those things are much, much better when there are 16 of you in the class rather than 128 or 512.
You wouldn't pay tens of thousands of dollars in tuition for lectures unless you were the kind of person that universities like Columbia try very hard to not admit.
Classes, however, are very valuable. You can ask questions when you don't understand things, you can talk with the other students to build shared understanding, you can get your homework graded and confirm that the things you weren't sure about on it were actually correct, you can take exams and test your skills. And those things are much, much better when there are 16 of you in the class rather than 128 or 512.