Both Messages and Mail both use Cmd+0 to bring the message viewer to the front. That's what the shortcut is listed as in the Window menu. Same with Calendar if you close the main calendar window
Messages does not have great keyboard navigation — I can't tab around like I can in Mail. I am putting this down to the fact it is a Catalyst app and they are a bit sloppy with consistency (not necessarily a Big Sur thing, as these were on Catalina)
But in Catalina Cmd-1 selects the inbox and full sidebar navigation including between accounts is possible with arrow keys. Nested lists can expand and collapse with left and right.
In Big Sur Cmd-1 just reopens the window with no selection. In addition you cannot navigate the full sidebar with arrow keys.
Combine this with the lack of visual indication of your selection and keyboard navigation becomes a struggle.
Messages does not have great keyboard navigation — I can't tab around like I can in Mail. I am putting this down to the fact it is a Catalyst app and they are a bit sloppy with consistency (not necessarily a Big Sur thing, as these were on Catalina)