Earlier quoted context omitted.
I'm not sure about that first part, some of the biggest games like gtav is an embarrassment in the concept of never trust the client.
GTA V is an exception because it's so easy to cheat in. I believe it's peer-to-peer with no verification among peers that what happened should actually have happened. It's basically impossible to secure that.
I suspect GTA VI may improve on these things and have centralised/dedicated/anti-cheat-guaranteed servers. Then again, it never impacted their profit margins so idk.