Under your server → Installer tab you'll find (overview: How the Installer works):
- Server version: Vanilla, Paper, Purpur, Fabric, Forge and Mohist in the version you want – see Change the server version and install plugins.
- Plugins straight from the SpigotMC directory (for Paper/Purpur).
- Modpacks from Modrinth and CurseForge, including NeoForge packs – see Install a modpack.
Plugins and mods only work with the right software: plugins need Paper or Purpur, mods need Fabric, Forge or NeoForge. Before changing the software, version or a modpack, leave the box for the automatic backup ticked.