China can integrate the warlords by taking the "subjugate the warlords" focus, which unlocks decisions to do it. It's very political power intensive though and I'm not sure if AI China is set up to manage it or if it just uses the warlords as puppets.
More commonly, you will see warlords take over China using the power struggle mechanics, which effectively turns them into the new China. Check who the leader of China is, if it's Chiang that's the original China, if it's one of the warlords then that warlord took over via the power struggle.