Yep, CatAndJ.
It is as you say.
However, you may note the note at the top of the "Recently Deleted" Folder: "Recently deleted items may be permanently deleted by your storage provider."
For iPads, and such, for files that were on iCloud (most things, but not all), that would mean Apple, on iCloud.
However, it looks like I have Google Drive files, showing up in my "Recently Deleted" Folder, as well. I expect that I would need to use Google Drive to try to permanently delete those.
I did notice that when I emptied the Trash, on my Mac, some of the files in the "Recently Deleted" Folder "vanished". (Much of the personal files on the Mac are on iCloud.)
I also found that when I deleted known iCloud Documents, from the "Recently Deleted" Folder, they seemed to stay deleted.
So. While I am not completely certain, it appears that the ones that are "sticky", that reappear, are those on other Cloud Services.
You can use "Info" (from the popup menu that appears if you hold your finger on a file) to see where the file resides. (All my undeleted files, that I checked, were on Google Drive.)