Most basic: page numbering on footer

Questions
4

Now I’ve set the context, but I want the page number at the location marked by X.

#set page(
  paper: "a4",
  margin: (left: 25mm, right: 70mm, top: 20mm, bottom: 28mm),
  numbering: "1",
  number-align: start,
  footer:   
    context{counter(page).display()
      [X — My Footer]
    #set align(left)
    #set text(10pt)
   },
  footer-descent: 10% + 15pt,
)

Instructions don’t tell anything about a placeholder code for the page number that I can put at X. (Everything would be much easier if we had a placeholder to introduce it in a string.)