DHIS Core documentation

Hello everyone,

A couple of months ago, I tried contributing to the core part of the project.
I chose an issue on the jira issue tracker ( Meta Attribute not accepting value for Option of optionSet) and tried to get an idea about where I should look in the repo. I looked at code in packages such as org.hisp.dhis.attribute (the DefaultAttributeService) and tried debugging but never hit it. I even tried jacoco code instrumentation to see what code is being run when I reproduce the bug. This didn’t work for me (do you happen to have a step where you rewrite the classes?)

I’d like to give it a second run. I tried looking for documentation (that I thought I once found on github), but only found documentation for apps and android parts of the project. Can anyone point me to any resources documenting the core part of the project? Some documentation I could read or watch to have a general idea about how things work and are organized in the backend.