We have a simple scenario but technologies(or implementation) have made things bit complex -
We have Sitecore as CMS for client's main web site and for business functionality we have Dynamics 365 Portal and we are using Azure AD B2C as identity provider for both.
We have heavily customised B2C custom policy to cater specific requirements.
We are using same custom policy in Sitecore and in Dynamics 365 Portals so sign in individually and works perfect without any issues.
With regards to SSO, we have kept the configuration as OOTB as you can find it here.
Only 1 or 2 user journeys are absolutely perfect as good as seamless journeys. In specific journeys we need user to be logged in both ends to make it work (we can force user to go to sign in page, but it is not required on all the pages).
In order to understand how real and proper SSO should work in Azure AD B2C, I have no idea or experience to relate this process to. so I am looking for guidance and help from this link https://docs.microsoft.com/en-us/azure/active-directory-b2c/active-directory-b2c-token-session-sso.
I found very good information from this answer(https://stackoverflow.com/a/50471360/2554489) about how B2C works in terms of signin and providing tokens process but it bit old question and things have changed a lot since then specifically UI and some of the operations.