i have a specific case scenario
i understand that a usa citizen (with hkg id) is about to enter china from hkg without a visa
if a usa citizen goes straight into china from usa, they would have to apply for a paid visa

does anyone have experience/knowledge if a usa citizen (with hkg id) flies from sin to hkg to shanghai (direct checkin at changi from sin to shanghai)?
does one need a visa ?
or can the usa citizen use the usa passport to exit sin but the hkg id to enter shanghai?