To the first question, it depends. If it's something you're building for a business, and its reliability is important, then you probably should understand code

To the first question, it depends. If it's something you're building for a business, and its reliability is important, then you probably should understand code you're copy-pasting. For a hobby project, it's probably not as important. The whole point of Chesterton's Fence is that there's wisdom encoded in the way things have been done. The previous guy who worked on the same module was probably somewhat competent, and in any case, the component was fulfilling business needs for all that time, so they were doing something right.