In the Finder, does the iOS device show up as a connected or mounted item on the left pane of the Finder sidebar? If so, try ejecting it.
Otherwise, You can try Force Quitting the Finder. You can do this by pressing Command + Option + Esc keys which will bring up a pop-up box with a list of running applications. Select the Finder to force quit. I would make sure you first unmount/eject all external drive first to minimize the risk of data loss with them.
Unfortunately it is not unusual for macOS to get stuck where some process just won't let go or macOS does not recognize that a process has finished. It can be very frustrating. I usually find it occurs when I'm trying to eject a drive/volume when I want to disconnect an external drive.
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