What is possible with the JIRA API

I want to start developing with the JIRA API. Therefore I watched and read a lot of tutorials but I couldn t figure out what is possible with the JIRA API.

So for example, is it possible to interact with the user via Buttons and if the button is clicked the plugin works with the data?
Or is it maybe possible to create Structure Boards with the API?

If someone could share a documentation about the JIRA API with all its possibilities, I would be very happy.