2 Bash commands to change strings in multiple files at once

Think about some situations when you need to change strings in text files in your Linux hosts. Depending on the case, you will simply change the file directly in your favorite text editor. But what if there are too many files? Too many changes? And on top of that, you want a more automated method to perform this task precisely, in a way that is also repeatable and somehow self-documented. Got your attention? OK, I’ll elaborate.