Part of the confusion is that they kept around the old clients and protocols. There is also a lot of confusion between the different apps and the platform.

Google Talk is the original XMPP based chat system and was known informally as Gchat or Gtalk. There were web and desktop clients. Hangouts is the "new" proprietary messaging system from 2013. Google Talk and Hangouts are integrated at least for text messages but Hangouts adds lots of features like group video chat. Google Talk provides XMPP protocol access for federation and third-party clients.

Hangouts app on Android could do Hangouts, local SMS, and Google Voice SMS.

I find it interesting that there is less confusion about Apple Messages even though it was renamed from iChat to iMessages to Messages. The app also supports both SMS and iMessage protocol. In addition, iChat supported XMPP and third party clients until was shut down.