The more you work with WordPress, especially if you employ tools like Composer and the like, the more you’re likely going to find certain libraries, tools, and similar things that form the foundation of projects on which you work.
Sometimes you may find them when looking through other people’s code, maybe you’ll find them when browsing GitHub, or maybe someone you work with will suggest a library (or a few) depending on a project.
Personally, I’ve had the experience of all of the above, and the more I work on building solutions for myself or others, the more I find myself setting up the same foundation for each project (which is typically a plugin of some sort).