Haha typical youtube
If there's a German word says "Sie", why does English eliminates the word "them"?
In this context, "Sie" means "you". In this sentence, this is the german formal form of you.
It is possible to say "Abonniere jetzt!" (informal), or "Abonnieren Sie jetzt!" (formal).
The original question remained unanswered. This is an imperative statement / command, without omitting "you" in English I'd say "You subscribe now!" which sounds foreign.
It doesn't remain unanswered; the premise of the question was false. It's not "they" in this context at all.
slapp like now