How to interpret a schematic in terms of input/output
It's all about pattern identification, and you get that by looking at simpler schematics and then building up from there. Generally, people pick design patterns from a fairly limited set of options and so once you recognise a few you can start to get a sense of what parts of the design do. After that, you can "ignore" those and concentrate your efforts on understanding bits you don't know.

I often find a challenge is the actual layout of the schematic. For example, in your SMPS schematic, it's clearly made to fit tightly into a rectangular space so some patterns are not in their textbook arrangement. Practice, practice, practice, and ask here when you need some help :)