Chrome and Firefox may use WebKit but have very different looks and feels and some different functionality. They’re not just “skins”.

They use a Cocoa feature called [WkWebView] (https://developer.apple.com/documentation/webkit/wkwebview) which is an embedded webkit browser. Any functionality that differs from Safari is not part of the actual "browser". It's just "skin" on top of it. (Bookmark syncing, etc)

WkWebView is a web renderer and partial navigator, it's not a browser.

My point is that people are really stretching the term "skin". Bookmark syncing, tab & window management, history management, security management etc. all are features of a browser, and they have a major impact on the UX of that browser. They are WHY there are still users of Firefox or Chrome on iOS! They're not "skin" features, which typically imply minor look & feel related items.

Not really different. They even share bugs.

If Safari team messes up their rendering engine, then iOS update, and it is broken for every iOS browser...

Do you use any of them regularly? I do. They're very different UX-wise.

My point is that users don't really care about the common rendering engine. It doesn't have much impact on the day to day UX.