Mods and plugins
Own tab. Plugins on Paper family, mods on Fabric, Forge, Quilt.
Mods and plugins are a separate tab, not inside Minecraft. On Paper, Spigot, or Purpur the tab is Plugins. On Fabric, Forge, or Quilt it is Mods. Vanilla shows Mods & Plugins and explains that it has no catalog folder.
Search the catalog and install the latest file that matches this Minecraft version. Plugin jars go in /plugins. Mod jars go in /mods. You can also upload your own jar or open the folder in Files. Restart after you install.
The catalog is Modrinth only in this first version: it does not pull Hangar or SpigotMC, and it does not auto-install extra dependencies.