static resource caching is easiest through the browser's http cache. As long as the background is configured with the correct http response header, the browser will cache automatically.

app, is directly stored locally, and each time you read the local file

webview caches static resource files like most browsers, and so does ajax, unless the request address changes.

another option

1. application cache
2. serviceWorker

is not an update. How do you know the update?
what is the concept of this update, background database update?
webview Android ios can set cached updates to request this. There is an identification handle