well yea i know what you are saying, but course descriptions are available online, and textbooks are available. some textbooks are available for free, or else you can buy a textbook, and if you read for example a whole calculus volume 1 and volume 2, then you know you are covering all that is taught in a few semesters of calculus. normally a class will follow what a textbook covers pretty closely. and with the internet there is so much resource available online, you could even go on a math science forum and say "these are all the topics i am studying, these are the books i am reading, will this cover everything i would learn in a 3rd semester calculus class?" and then someone who knows more can say "yeah almost except there is this one thing you also need to study". it just means you have to take initiative yourself. it is 100% doable.
if you miss concepts then you probably just aren't reading the book thoroughly enough, or you can ask online on a math forum.
but for resources outside the classroom, for math i disagree with you, but for science i agree with you that it can be helpful to have access to a lab, it just depends what you are studying and what resources you have available.