how would I go around learning about different design patterns and approaches to problem solving if I don’t know about their existence in the first place?

Read, and try things. A lot. Read books, read articles, read forums, go to conferences or watch videos from conferences. And try things - do small projects at home, do large projects at home, work in multiple projects at work, if you can. In all these projects, try various things. See how good or bad they are.

There really isn't any shortcut for this. This is why experience is so valuable and sought after. There is no replacement for it.

for example say I need to create a system for inserting a large amount of data from files into the db

Read about how others have done it, and how it worked for them. Ask colleagues, if you have some which did this before. Remeber how you did it (if you ever did it before), and how it worked out for you. Try some prototypes, and see how they work. These are the strategies most people use.