This is how PWAs work on smartphones. I recall coming across some electron alternatives that use the system webview and those are able to generate binaries hundreds of kilobytes in size. But you lose out on access to many of the modern APIs due to Safari there.

This is standardized for many years and called PWA. There are ways to interact with native helper utilities as well (simplest is just run http server on localhost), but not for mobile apps.

For mobile apps you can use webview component which will use system browser. You don't need to ship the entire browser (actually you can't even do that on iOS).