If I was asked a question on something I just saw I'd probably have already forgotten... I unfortunately am a rote-learn guy.
Someone will rattle off some process to me on the phone and yeah... these kind of scenarios. Way to train yourself... maybe puzzles idk. I do have really bad short term memory... I'll record a gif of something and in a couple seconds I've forgotten what it was about.
Here are some ways to do it:
Breaking concepts into parts and only focusing on understanding one small part at a time. This acts as a 'foothold' for understanding the entire concept, similar to how a foothold in a mountain helps you climb part of the mountain.
Identifying and then ignoring irrelevant parts of the concept (trimming the fat).
Analogizing the concept (or the small parts of it/footholds) to concepts you already know.
Of course there are probably more ways to compress a concept. If someone knows any please do share.
The idea is to work with small concepts in your 'cache'. If a concept is too big you won't efficiently learn it, especially if your short term memory is bad like mine. So, you can break it down into smaller parts and learn each of those, and then combine them at the end. Or you can point to a concept you already know that is in your long-term memory (analogizing).
Software-development speaking, I feel you, I'm also slow at reading requirements. But I'm slower if I start with over-generalized use cases, rather than going through concrete scenarios.
So, if I were you, I would make a list of subjects that are interesting and select books that are fundamental to the subject. Example, if you want to understand calculus then you better have a good grasp of math, algebra and trig. Learning by memorizing is only part of the answer. It's hard to advance your knowledge if you don't understand what you are reading. So you have to take the time to make sure you understand the subject by taking the time to answer problems.
If you want to be quick at understanding interfaces then take the time to understand how they are designed. Overtime you will be quick to pick up new interfaces. There are no quick answers. You have to put in the work.