There's a bit of a distinction in the Minecraft world between "mods" and "plugins". Even though to the outside perspective they're the same thing, the terms are used for two different communities/ecosystems, basically.

"Plugins" are for bukkit-based compatible servers, like Spigot and Paper (and Folia, with some modifications).

"Mods" are more for forge, sponge, etc servers.

So you can kinda pick which direction you want to go. But the Paper project has a ton of API documentation and a few Github repos to even get you started writing your own plugins. The Discord server has a dev help channel with people happy to answer any questions.

The Sponge team has a similar setup as well if that's the route you'd like to go.