I think they probably have data miners that just pull your email contacts in to their own cloud and shoot the spam from there. What they do is take your entire address book and send spam to everyone in that contacts list, but rather than having you as the sender email they have the other contacts as the sender so friend A will get spam that appears to be coming from friend B instead of from you. Google probably has AI that spots a bunch of spam being reported from your particular GMail contacts list and told you someone hacked your account to get that contacts list most likely, but no way to tell for sure. VPN and spoofing have pretty much made it all but impossible to figure out where emails, texts, and calls come from without major investigating..