Hi,
A plugin is a piece of code that interacts with X-plane by reading and/or writing datarefs. It can do anything you want, it's not just about joysticks and you don't actually need any additional X-plane plugin to use a joystick.
Most of third party plugins can be downloaded on X-plane.org. You just need to unzip the downloaded file and follow the included instructions to install them.
Be aware that Laminar Research does not provide any support for any crash a third party plugin could trigger on X-plane. If such a crash happened, you could always unsinstall them or even disable them from X-plane plugins menu.
If you are looking for a plugin to enhance your joystick management, you might be interested in Xassign or even SPAD.neXt. Yet I have tested none of them.
Regards,
Guillaume