Here is a list of resources from around the web that I have found helpful to share with customers when they ask me how they can learn more about Azure Active Directory.
Protocols and Token Flows
Often when I am asked "When do my users have to do MFA?" I always have to answer with "That depends" which is a bit of an ambiguous answer. However, in order to understand when someone might be asked for additional proofs, you need a basic understanding of what protocols and what token flows are in use depending on the use case.
I usually recommend people view the sessions from MS Ignite 2018 which provides a good foundation for understanding the modern protocols in use, and how the flows are enacted per the usage scenario.