Need information regarding configuration required

SPID
1
  1. The documentation here mentions that we are supposed to provide authPageUrl, callbackPageUrl and queryStringParams. What is the purpose of these, can you please elaborate in detail?
  2. We are supposed to add Identity Providers ids (example aruba, infocert etc). How to obtain these ids? Are they unique for everyone who wants to integrate SPID in their app or we need to contact these Identity Providers in order to obtain ids?