You pretty much got it! One nitpick, your example of Lemmy.unitedStates is not NECESSARILY a federated instance, just an instance. It may or may not be federated with other instances.

De-federated instances lose communication to/from the instances that said "im gonna de-federate from these Nazis", so again you pretty much got it right :) not silly questions at all! It's a new concept to many people and I promise you MANY people have and will ask these "dumb" (but not actually) questions!