Mods fundamentally change the game — adding new blocks, dimensions, machines and creatures. Unlike plugins, mods must be installed both on the server and on every player's client.
1) Pick the right software
When creating the server, choose Fabric, Forge or NeoForge as the software. Check which loader your desired mod supports on its page; Fabric and Forge mods are NOT compatible with each other.
2) Upload the mods
In the panel's Addons tab you can search Modrinth and CurseForge, or manually upload .jar files into the mods folder from the Files tab. Don't forget the mod's dependencies (e.g. Fabric API).
3) Client-server matching
Every player connecting must install the same mod list and version in their own Minecraft as on the server. Server-side-only mods (some performance/optimization mods) are the exception. A mismatched mod list errors out on connect.
Running a prebuilt modpack
Download a modpack .zip from CurseForge or Modrinth, upload it via Files → Upload in the panel and use the 'extract' option. Mind the pack's recommended RAM — big packs want 6-8 GB or more. You can raise your RAM with a package upgrade.
RAM and performance
Modded servers use far more memory than vanilla. As a rule: a small mod list wants 4 GB, a mid modpack 6-8 GB, big 'kitchen sink' packs 10 GB+. For performance, optimization mods like Fabric + Lithium make a big difference. Note: mod software is available on Pro and higher packages.