After a download completes, in the downloads dropdown (Ctrl+J), if you right-click on an item you get a context menu.
One of them (as per sreenshot below) is "Copy download link"
But it would be very useful to have an additional menu item here "Copy path to downloaded file" which will place the full drive/folder/file path to the saved download in the clipboard.
The alternative is to click "View in folder", then Shift+RightClick on the file, then "Copy as path" in file explorer. But this can lead to a proliferation of explorer windows in the taskbar.