Basically all addon documentations and some eclipse contents like concepts and parts of the development docs.
See: GitHub - openhab/openhab-docs: This repository contains the documentation for openHAB.
You can get an impression of it on the link i posted below.
It is roughly documented here: GitHub - openhab/openhab-docs: This repository contains the documentation for openHAB.
When you switch to the final branch in the github docs repo
you can reach the 3 mentioned files from the linked paragraph.
There are not that many comments in the code, so its maybe hard to read if you are not already familiar with bash, maven/pom and groovy.