It is using HTTP, just doing it through browser APIs[0][1] so in this case the server layer is the service worker but it's still happening through HTTP means. So the client and the server are both running in the browser, the only difference is that this "virtual" server can only be accessed by the same browser session. They are very powerful APIs as you can intercept the network layer and do all sorts of hacks, like the OP's project.

[0]: https://developer.mozilla.org/en-US/docs/Web/API/Response

[1]: https://developer.mozilla.org/en-US/docs/Web/API/Service_Wor...