Learn French: Est-ce que vous visitez la ville maintenant ?

Est-ce que acts like a built-in question mark at the beginning of a sentence. It signals to the listener that a yes/or/no question is coming. If you take it away, you are left with the regular statement vous visitez la ville maintenant (you are visiting the city now).

The English translation says "Are you visiting", but I don't see a word for "are" in the French sentence. Why is that?

In French, there is only one present tense. The single word visitez covers both "visit" and "are visiting". You never use the French word for "are" to translate action sentences like this!

Could I use this exact sentence to ask "Are you visiting Paul now?" by swapping the city for Paul?

No, because the verb visiter is only used for visiting places, like a city or a museum. If you want to talk about visiting a person, French uses a completely different phrase (rendre visite à).

I know vous can mean "you all" or a formal "you". Which one is it here?

It could be either! Without more context, vous visitez could mean you are asking a group of friends if they are visiting the city, or you are politely asking one stranger. The grammar stays exactly the same for both.