Quite a powerful concept, in particular the offline ability
○ Store site/app data locally so that whatever has been downloaded so far can still be access if you go off line. Potentially and changes you make offline can be retained and uploaded way you get access back - but this can of course have some synchronisation issues!
Push notifications can be sent by the server, if a browser/app registers itself as a client
The service worker can also communicate with the webpage/app using messages. The SW and page/app run as separate entities as a security measure, but the webpage can register with the SW as a client (each tab can do this individually) and then they can communicate via messages. There is a Subscription in the RP code to manage receiving SW messages
If you change the sw.js code, need to close browser and start again otherwise the old service worker will hang around and stop the new one from controlling the page. https://stackoverflow.com/questions/48859119/why-my-service-worker-is-always-waiting-to-activate