Plugins
Minecraft Plugins are a type of Server-Side mod, which means they can't change anything on the client that isn't within what the client would expect from a Vanilla server. There's a lot you can do with that, but it often means you can't do things like add blocks, items, or entities to the server with the same solidity as you experience with Forge or Fabric.
The benefit is that Vanilla and lightly modded clients can connect to your server with no changes to their client, and your world can easily be upgraded to new versions of Minecraft, or even run without your plugins if you don't want them anymore.
There are thousands of plugins made by community developers, but we've written a few articles for our favorites.