How do you learn backwards?
As I start reading research papers I'm starting to come across a lot of topics that I don't know. I like learning subjects using a bottom up approach meaning I start from the basics and work my way up. This way I get the big picture and I feel like I understand things deeper. So for a while what I would do is get a book on that topic and start studying it. But reading a whole textbook can take a lot of time and even worse sometimes I have to read one book as a prerequisite for another book.
I think I need to change my approach and start one that is top down. How do you guys do this especially with more advanced topics? Whenever I try this I always fail because I just don't understand until I go far back enough where I'm essentially doing a bottom up approach again. Is this how research is usually done? I can't tell if I'm approaching this wrong or if I just have a weak foundation.