You are allowed to use it for private use cases, but you must not upload it as your own. Don't use any files outside of this project for your own plugins.
You are allowed to correct errors, build it yourself, make extensions and sell them.
In this repository you can find all extension, the extension .json file and version.json.
If you are looking for the source code, go to source/imporvedFactions. In there you will find diffrent projects. The ImprovedFactions old plugin will be without anything.
If you want to see the current new plugin, go to ImprovedFactions_new. Lite is currently empty.
If you want to write your own extensions take a look at the existing ones. They are in source/extensions. If you need help, join my discord