This deck offers a nice presentation of what is NSX Federation and how it works.
A very similar deck was used at VMworld 2020 session VCNC1178D here and watching is a nice option as it gives "voice over" the deck.
Federation demos are also available here.
This ToI may be updated in the future so always check you have the latest version.
. NSX 4.0-4.1 Federation 101 ToI version is 1.0 done on 10/30/2023.
. NSX-T 3.2 Federation 101 ToI version is 1.1 done on 08/26/2022.
For deeper information, we also offer the "NSX Federation Multi-Location Design Guide (Federation + Multisite)" here.