There are two options for saving another user’s content. Both appear at the top of the deck page, across from the title.
Bookmark creates a shortcut to the original deck in the chosen location. If the deck’s owner makes changes, you will see them. You can’t make changes.
Edit a Copy creates an identical deck that you own. This copy is entirely yours. The original owner’s changes will not apply to your copy and your changes won’t affect theirs.