To use these plugins, copy any of these folders to the addons/ folder in a Godot project.
For example, the path would look like: addons/custom_node
Plugins can be distributed and installed from the UI. If you make a zip that contains the folder, Godot will recognize it as a plugin and will allow you to install it.
This can be done via the terminal: zip -r custom_node.zip custom_node/*