. Good morning, Beachboy.
(1) Yes, this is, indeed, a favorite topic.
(2) I think that you understand it well:
(a) My favorite fruit (singular) IS oranges.
(b) Oranges (plural) ARE my favorite fruit.
Onr book says that even native speakers may want to say:
My favorite fruit are oranges (because their minds are thinking about
"oranges," not "fruit," but they realize that it is "bad" English, so they
follow the rule. )
Have a nice day!