Which of these is added to flip cocktails?

I read the explanation, but still don't understand. Definitely not a drinker.

Rai, doesn't matter if you are a drinker or not... I found it a bit hard to follow too?

I'm not a drinker either, but it seemed easy to understand. Originally, frothy drinks were made by heating beer, but now the froth comes from the addition of eggs. The last paragraph explains the difference between eggnog and a flip.