Slight nitpick I know but decrypt here is the not quite the right word. The communications are still encrypted in flight but they are necessarily decrypted (by WhatsApp) on the end (your device). This unencrypted file is then provided to drive if you select back up with Google drive in WhatsApp.

I'm in two minds about this one. I'm not really a fan of Google but realistically we're talking about backing up messages from a Facebook app. It is also possible to opt out and I know non technical people who have done so on the basis they don't want "more Google".