That is incredibly weird, and should be impossible. You're positive that you don't have any datapacks, or any other plugins? Is there anyone else with OP who could be pranking you?
Also, the way you add Alex's Caves on paper is simple: you don't. It's a forge mod and requires a forge server. Datapacks like terralith, however, can just be added to your datapacks folder. It's a vanilla feature
I don't mean to argue with most of your points, but regarding opposability: They have a video where they show it being used, and the idea is that you use it to hold something against your wrist area, so that you can manipulate the top part with your hand. They don't use it alongside the other fingers, it's just a complicated way to hold things still