I heard (from a research professor!) that the best way to learn real-world large-scale design patterns is to work in industry. Because at a big company, you are working on a real-world large-scale system. So I would recommend, if you have the opportunity, try getting an internship before grad school.

Of course not all companies have the best practices, but in my experience most do. If you decide to find an internship, ask about design patterns / code reviews in your interviews.

Alternatively, you could read up on good design patterns and practice. I've heard people recommend Uncle Bob's "Clean Code" and "How to Design Programs". Also see https://news.ycombinator.com/item?id=25299547.