If you want to learn how Org mode works, start by reading outline.el

then read Dominik's outline-magic.el

then see how far you can get with your own customizations on top of that to augment, e.g. plain markdown text

you may find you're doing well at that point and don't need to deal with the 100k+ line beast that is Org, nor its transitive dependencies

at that point you will really understand (that part of) Emacs, and will no longer see Org as "a discrete thing", but just "some code someone else wrote" that you can take or leave