For Visual Studio, Windows (11) English Version German Keyboard the default seems to be:

ALT+.
ALT+,

The setting can be found Tools -> Options -> (in dialog) Environment -> Keyboard ->

Edit.NextSuggestion
Edit.PreviousSuggestion

Or simply type in the search box: "suggest".

Note: Sadly first trying to type "copilot" did not work, since this feature does not have that in the name (others have).

Note: I know this question is for Mac, but I found this first, even though I was looking for a windows solution, so here it is.