How to safely clear the trash as sudo?

I have files with elevated permissions in the thrash folder. Wiping the path with rm doesn’t seem safe, if some hidden files are stored there.

The most promising solution is this answer, but I don’t understand what the full command is.

install trash-cli and execute:

sudo HOME=/home/<hour-home-dir> trash-empty

trash-cli is in the Fedora repos.

UPDATE: Please also consider giving guidance for clearing the trash folders of mounted external drives (in /mnt/backup/.Trash-1000/, and so).

Files in trash is located at $HOME/.local/share/Trash/files directory (I can’t verify this since I don’t have one right now, but it is what I remember and also mentioned in the answer you linked).

sudo rm "$HOME/.local/share/Trash/files/put-your-filename-here"

Neither Nautilus nor ls lists the same files as displayed in the trash:/// path (as shown in the Nautilus file explorer’s “trash” tab.

Does this display anything?

sudo ls -l /root/.local/share/Trash/files

ls: cannot access '/root/.local/share/Trash/files': No such file or directory

UPDATE: a mounted drive, the data is stored in /mnt/backup/.Trash-1000/.

That sounds like a misunderstanding.
Removing hidden files is not different from others.
It should work just fine:

sudo rm -f -R ~/.local/share/Trash /mnt/backup/.Trash-*