Types of Add-ons
Client
Client add-ons add new features to the client. You don't need to install client add-ons on the server.
A single file supports both Forge and Fabric.
Installation
Add the add-on into the ~/mods
folder.
Universal
Add-ons marked as universal are server-side only.
You only need to install it on the server and the feature will work for all the players with Plasmo Voice.
All universal add-ons work on Spigot, Fabric and Forge.
Some can be also installed as Bungee or Velocity plugins.
On Fabric or Forge
Add the add-on into the ~/mods
folder.
On Spigot, Bungee or Velocity
Add the add-on into the ~/plugins
folder.
Note that you can install the add-on on Bungee or Velocity only if add-on description says you can.
Other
Add-ons not marked as universal or client may work differently.
For example, they can be exclusive to a certain platform, or have different files of a different platform.
Read the add-on description to figure out how to install it and what platforms are supported.