One of the ways to “get stuff in” for me has been to try to Print…, and then, from the menu of options of what to do with this PDF, to select "Send to Devonthink.
This is particularly useful when I’m viewing an email, e.g. within the Gmail web interface.
It might be “a Big Sur thing”, but I no longer see DevonThink in the menu (screenshot attached).
(I’m hoping there is a config setting somewhere that might fix this )