To try to give you clarity. The "true copy/original version" of every comment you make is stored on your server. When you click the Fediverse icon (the rainbow star thing) link to your comment it will be of your server, and other server user's content will be on their own domain when you click on the icon within their comment.
Copies of this comment are stored on every server that accesses that post thread that federates with your server. Any changes made will propagate when the copies sync up to the original, including deleted status. If the original server goes down unexpectedly or defederates, the copies on other server no longer sync and remain as they are.
Technical aspects aside, when you post in a community on another instance, you need to follow the rules of that instance. However as a user of your home instance there might be certain expectations placed on your conduct. (Here it seems like don't upload large files is the main thing.)