Is it common for people to supplement learning with outside resources like books/courses?

• I think in this age of so many online resources, probably yes. However, I don't think it's a good use of time most of the time.
• Books/courses are very structured, so they tend to be more generic and high-level. At the end of the day, you work for a specific company, so yield is going to be low.
• Also, if your goal is to build technical proficiency (e.g. I want to get better at running queries or writing this code), you mainly learn by doing. The opportunity cost of reading is not you're not tinkering.
• Big Tech has the worst yield when it comes to outside learning as the stack is so custom, and in general, the bigger/older the company, the worse outside resources get. I think outside learning is best for startups, but even then, I like side projects a lot more. That's how I really supplemented my learning at Course Hero.

I find myself getting quite deep in very specific areas at work, so sometimes taking a step back and supplementing with a book that provides a higher level overview and broader exposure is quite helpful in strengthening my mental models of systems.