You don't need to download jars and upload them by hand. Minecraft → Plugins / Mods searches Modrinth (mods and plugins) and Hangar (plugins) for you, and only shows what fits your server.
- Open your server in the panel and go to Minecraft → Plugins / Mods.
- Search for what you want. Results are filtered to your Minecraft version and your loader.
- Click Install. The file is downloaded and its checksum checked before it reaches your server.
- Restart the server so it loads.
Mods or plugins?
That depends on the server type you created:
- Paper, Purpur, Folia and Spigot run plugins. Players don't need to install anything to join.
- Fabric, Quilt, Forge and NeoForge run mods. Players usually need the same mods installed to join.
- Vanilla runs neither.
Something only on CurseForge
The tab doesn't search CurseForge. Download the jar from CurseForge yourself, check it's for your Minecraft version and loader, and upload it to the mods or plugins folder over SFTP.
Jars you added yourself
If you uploaded jars over SFTP, click Identify. Each one is looked up on Modrinth, after which it gets its proper name, an icon and update checks like everything else.
Client-side mods
Minimaps, shaders and most HUD mods only run on your own PC. They don't belong on the server, and can stop it from starting. If it won't boot after adding mods, see why won't my server start?.