You seem to underestimate the amount of modding that vanilla Minecraft servers have, such as Wynncraft or Hypixel Skyblock. They turn Minecraft into a completel

You seem to underestimate the amount of modding that vanilla Minecraft servers have, such as Wynncraft or Hypixel Skyblock. They turn Minecraft into a completely different game and most of the changes are server side. Also, Fabric and Forge are not modding APIs. They sometimes provide extra APIs on top of the base game to make mod compatibility easier, but you are very much supposed to just mess around with the Minecraft codebase directly and directly reference Mojang classes in your code. If all you want to do is add a new block in Fabric you don't touch any Fabric API.