They're all separate chat services, yes, but the problem is the use case and differentiating features for each one isn't clear.
As I understood it, Voice was dying on the vine, and then google pushed everyone to Hangouts (to replace the built in SMS app on the phone anyways), but now Hangouts is suggesting to use other apps for SMS (but it still can anyways), so do I use GVoice or the built in one (answer: depends on the phone number you want the recipient to see). But now Allo came out, and that's somehow kinda like Hangouts but not? It has not-SMS chats like Hangouts, so it's just a hangouts replacement. But wait, Hangouts also does video chat - how's that different from Duo?
Google's product messaging is probably the most confusing in the entire technology industry.