SmartThings and Google Weave--technical questions

So SmartThings is now also weave compatible, what exactly does this mean? I know weave is a communications protocol and presumably is what enables integration with Google Home/Assistant, but how does SmartThings bridge the gap between it and zigbee/zwave. Can weave work locally without ST cloud services or is it a cloud service itself that just communicates with ST cloud to cloud?

I imagine its a cloud integration. As weave uses Thread as a local protocol not Zigbee or Zwave.