How to get the front end and backend code, REST of existing JIRA software?

Hello,

I am very new to jira plugin development. I have to develop plugins and addons for JIRA. In my work there is a JIRA software used in the work, and I have my own JIRA cloud instance, so I am wondering how to use the backend REST API and server of the JIRA software used in project from my jira cloud instance. I know that there is Atlassian connect express framework, but I do not understand how to have the backend and frontend code structure of the already existing JIRA app in project to develop additional functionalites? Please help, I have no expierence in JIRA development

Thank you

Are you working with Jira Server, or Jira cloud? Since you have tagged both, it’s a bit confusing what you want to achieve.

Or does your app need to work with both?