After a few requests for advice on how to structure plugins, I’ve created a tutorial that really breaks it down. This is geared toward those who aren’t yet sure what-should-go-where, and those who would like to learn to better organize their work in a consistent manner. This isn’t a hard set of rules; it’s just a pile o’ guidelines and better practices to help you get off to a good start. There are many ways to structure your plugin; this is one.
https://codepotent.com/plugin-structure-classicpress-plugins/