This deck offers a nice presentation of what is NSX-T 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.
Note: This document may be updated in the future so always check you have the latest version. This version is 1.0 done on 11/19/2020.
Note2: For deeper information, we also offer the "NSX-T 3.1 Federation Multi-Location Design Guide (Federation + Multisite)" here.