By default, every bookmark goes into one reverse-chronological list at x.com/i/bookmarks. Folders split that list into named groups — reporting, recipes, a project — so you are not scrolling the entire history to find one post. The post stays in the main list as well. A folder is a view, not a second copy.
That distinction matters. Deleting the original post, or the author locking the account, empties the bookmark whether or not it sat in a folder. Folders organise the pointers. They do not archive the text. Folders are private, like the rest of your bookmarks. Nobody else can see them, as explained in can people see your bookmarks on X?