Sorry about the ambiguous title.
In Master Page Editor I create a new Master Page
I am starting with the Left page and add a Text Frame to the top left to populate it with {@PageNumber@}.
I edit the text to the size of 10 points and give it a left alignment.
Now I copy the Left page to the Right page by clicking L>R.
The page number frame is mirrored to the Right page at the outside right.
Still the alignement of the content of this frame is to the left…
If I manually change it to be right aligned I notice, the Page number Frame on the Left page is suddenly right aligned, too (without me having clicked L<R).
How do I get the Page Numbers to be aligned to the outsides of the pages?
One would hope one could manually unlink the two fields just as one can now unlink grouped dynamics, but AFAIK this is not (yet) possible for fields “copied” (not “mirrored” actually) between Master Pages.
may be I should not be starting with an empty page, instead use the Default one
and change it (as on the Default Master Page the Page Number Frames and their content is mirrored).
Have you tried using the Page Number text style?
brilliant! This works 
You can, of course, modify that text style to whatever size and font you like, but I believe it has some unique magical code that makes it align properly. Either that or there’s a setting I’ve not looked for in whichever of the text styles dialogs found on the Engrave menu.
The Page number paragraph style has an option that says it goes to the outer edge of the page. There’s another option that says inner edge, which can be useful to add headers that won’t collapse with a centered header.
Marc is, of course, correct. I’m nowhere near a computer or Dorico