I am wondering whether it is possible to incorporate Affiliate widgets - more specifically accommodation locations from websites like Booking.com - into a separate map layer on an ArcGIS online map? So the user can select to view location, and turn a map layer on to show accommodation locations? And the user could click on a location and then a separate web page could open in the Affiliate website to show details, prices etc? Experience Builder would host other web pages such as tourist attractions, events happening etc.
If this third-party has a publicly available layer on ArcGIS Online or a compatible REST endpoint, you could use it to bring their data into your webmap. If they do have a published layer like that, they will likely have included links in the popup template or you could try to build such a link yourself in the Webmap Viewer.
You may also want to look into ESRI's Places API. It is there attempt to build a Google Maps competitor.