I am trying to connect a custom domain to the subpage of an existing website hosted via Firebase Hosting. Just like in website builder sites who allow users to 'connect a domain'.
Should I connect the new custom domain to Firebase's name servers, and then reroute to the specific subpage?
Or is there a separate standardized approach which lets a website publish content to custom domains?
You cannot map your DNS records to a subpage because it just map IP addresses to hostname. The solution for you is to redirect requests from source page to your destination page.