icon: LiNetworkSpigot / Paper servers are 2 out of the many plugin loaders you can use. These two allow you to add plugins to your Minecraft Server. Minecraft Plugins are a way for you to modify your game by using already made in game features. This means all of the plugins you use are server side, meaning players will not have to install anything additional to Minecraft in order to play on your server.
Forge and Fabric are the most known mod loaders. These allow you to install modifications not usually built in to the game, to give your server a unique element to it. However, Forge and Fabric servers require the player to also install some of the client sided mods.
You can install Minecraft plugins on spigot, found at https://spigotmc.org. When you have downloaded the plugin you want to use. Head over to Atomic Host Dashboard. Select the server you want to add your plugin too. and press "Manage", next, click the "Files" button on the left hand side. and then you should see a folder called "Plugins", here is where you will upload your .jar file you just downloaded.
Note: To "enable" plugins on your server, you will have to restart it after uploading it to your plugins folder; Server plugins may also generate a folder along side it, in here will most likely include configuration files, for you to customize elements of the plugin