What is plugins in Jenkins?

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1. What Is a Plugin in Jenkins?
A Jenkins plugin is a self-contained Java archive (.hpi or .jpi file) that extends Jenkins' core functionality. Plugins allow Jenkins to integrate with tools like Git, Docker, Maven, Slack, and even custom steps in your pipeline.
They’re the backbone of Jenkins’ flexibility — you can add build triggers, post-build actions, UI elements, pipeline steps, and more.

2. Where Are Plugins Stored?
Jenkins stores plugins in the following location on the controller (master) node:

$JENKINS_HOME/plugins/
Each plugin is stored as a .jpi or .hpi file.

When installed via the web UI or CLI, Jenkins may rename .hpi to .jpi.
You’ll also see .jpi.pinned, .jpi.disabled, and .jpi.version files depending on plugin state.

Example on Linux:
/var/lib/jenkins/plugins/git.jpi
/var/lib/jenkins/plugins/workflow-step-api.jpi

2 Code Reference Example: Custom Pipeline Step Plugin

You may find details about Jenkins plugin development in this tutorial
https://www.velotio.com/engineering-blog/jenkins-plugin-development