Learn Spanish (Latin America): Alguien está revisando la bolsa.

Yes! Just like "someone" or "somebody" in English, alguien is always treated as a single person grammatically. That is why it is followed by the singular third-person verb está (is) rather than the plural están (are).

Does revisando mean "revising"?

Not exactly. While they look similar, revisar usually means "to check," "to inspect," or "to search." If you want to talk about "revising" schoolwork for an exam, you would use a word like repasar (to review) or estudiar (to study).

Why are we using está revisando instead of just revisa?

You could simply say Alguien revisa la bolsa to mean "Someone checks the bag," but está revisando emphasizes that the action is happening right now in this exact moment. This is formed by combining the verb estar (to be) with the -ando ending, which works just like the English "-ing" ending for -ar verbs.

I've also seen the word bolso. Is it the same as bolsa?

They are closely related but slightly different. La bolsa usually means a plastic or paper shopping bag, a trash bag, or a generic sack. El bolso generally refers to a personal handbag or purse.