Thanks for the detailed answer. While it is not strictly related to auth0, I didn’t find any proper discussion board on the net or examples covering this scenario. But I think it is very common: SPA + backend for sure is, and most of the time you also want to integrate with facebook or spotify etc. Probably it is worth an article on Auth0’s blog or documentation.
Another question: is auth0 (as a service - aside from client addons) involved in any way / can be leveraged in any way? It seems to me the answer is NO, unless the 3rd party service is used for linked/social login too, right?