What you learn just to pass gets rented. What you learn because you wanted to gets kept.

Material taken on for a deadline is held on loan. You return it the moment the exam is over, because nothing about it was connected to anything you cared about and there was no reason for your mind to keep paying the storage. What you chose yourself attaches to other things you know, and that web is what makes knowledge stay.

You cannot want every subject, but you can usually find one corner of it that is genuinely yours. The history of the thing, the application you would use, the part that annoys you. Study outward from that corner and a surprising amount of the rented material becomes owned.