1. Start here https://developer.atlassian.com/jiradev/getting-started/gett...
2. Install the SDK (it has some pre-reqs) and build Hello World https://developer.atlassian.com/docs/getting-started/set-up-...
3. Bone up on Plugins2 a bit more https://developer.atlassian.com/jiradev/jira-platform/buildi...
4. API Reference https://developer.atlassian.com/jiradev/jira-apis (I recommend the REST APIs + Webhooks wherever possible).
I'm really keen to talk with you about your experience going through that - there's a lot of history and we're aware we need to clean up the journey.
_* If you were developing for JIRA Cloud you'd be using Atlassian Connect.
Like there's no one JIRA API via REST or something?
Thanks for the reply and the links. Really appreciate it!