It wouldn't hurt to search for testing courses too (it seems you're trying to absorb as much as you can 👍).
Not all code is easy to test (automatically) but if (when) it's well designed, you should be able to add (unit) tests. Then you can make changes (improvements, bug fixes) and know for sure that what your testing still works.
It helps to understand (unit) testing, because testable code needs to be well designed, and well designed code is easier to test. So, knowing a thing or two about testing will help with design choices.
Others will hopefully post some links, I'm not organized enough to have them at hand.