Thank you for the information, Hans!

Could you please also shed some light on the relation/difference among \{setup,switchto,use}bodyfont?

There seems to be not much documentation on this. Can one conclude, from the wiki and the source code, that they are mostly the same, except that \switchtobodyfont is restricted to the content area, and \usebodyfont is a helper to call \setupbodyfont in the beginning?

If this is the case, is it recommended to always put a \setupbodyfont[...] in the beginning, even if it's just a \setupbodyfont[modern]?

One reason might be that the following example somehow enlarges the page number in the header, intuitively contradicting what is claimed by the wiki: