A PWA is just a regular web app which attended the best university and mastered user experience
We all know native apps can open without an internet connection because when we download and install them, critical resources like UI components and some data are stored in the mobile. well, this is exactly what happens in a PWA as well.
The PWA stores HTML files, CSS files and images in the browser cache and the developers can fully control the network call. All of these are being achieved by Service Workers.