Do with singular or plural?

Could you please clarify for me the correct usage of "do" when referring to a singular or plural subject? For instance, if I'm speaking about one person's actions, would I say "does he do it?" Or, if I'm referring to multiple people's actions, would I say "do they do it?" Additionally, could you provide some examples to further illustrate the distinction between using "do" with singular and plural subjects? It's always good to be precise in language, especially when discussing complex topics like cryptocurrency and finance.