Modded plans run Forge or Fabric. Every player needs the same mods installed on their PC (use the CurseForge or Modrinth launcher).
One-click modpacks: open the Modpacks tab on your server in the panel, search for a pack (CurseForge and Modrinth), and install it. The server is reinstalled with the pack's mods and configs.
Manual mods: stop the server, upload .jar files to mods/, and start again. Make sure each mod matches your loader (Forge or Fabric) and Minecraft version.
Versions: set MC Version and Forge/Fabric Version on the Startup page, then reinstall to switch. Big packs need RAM: 8 GB covers most, the heaviest (ATM, GTNH) want 12 GB or more.
