It's quite easy but it depends how you want to do it. Would you want to take an existing PWA codebase and deploy it in the best possible way to mobile, or do you want to wrap the existing externally hosted PWA? Both are possible but the former will be more likely to be accepted into the app stores.

Capacitor can be added to an existing web app codebase and acts as a library with cross-platform APIs for functionality like Camera/etc and that would be compiled/linked into your app like any JS library. But it can also be used to wrap an externally hosted app by changing the server.url config value https://capacitorjs.com/docs/config