Plugins are mods used by modified Minecraft servers like Bukkit or Spigot. As you may knew the Minecraftcraft server is a Java application, the plugins for Minecraft servers are written by Java too.
Minecraft Java Editions have Pi API to support Respberry Pi. The RaspberryJuice Plugin(https://dev.bukkit.org/projects/raspberryjuice) is program with the Minecraft Pi API. The plugin listens to port 4711, and accepts most of the Minecraft Pi commands. For more details please visit https://wiki.vg/Minecraft_Pi_Protocol
The Spigot Minecraft server zip file you can find from my site already include the RaspberryJuice plugin. minecraft_spigot-1.15.2.zip
If you want to install it yourself, you could download from the from RaspberryJuice Plugin(https://dev.bukkit.org/projects/raspberryjuice), and put in the plugin folder
mcpi or mcpi-e is a Python Module could be used to comunicate with the RsaperberryJuice on the port 4711. So if you install the mcpi-e Python module, You can send your command to the Minecraft server. Please viewthe diagram below to understand how it works:
