When I logout in my browser (Firefox) from Google account, I can see that some pages in other tabs (not all belong to Google ecosystem) are refreshed and I guess that some logout notification was delivered by something to them. How does it work?
It's going to depend on how each application has implemented Google's API for user authentication. Most likely there's some Websocket or AJAX calls going on in the background polling Google to see if you're logged on. When it finds that you're not the calling app changes your status.