All web browsers on iOS are required to use WebKit. I think they’re actually restricted to a webview and don’t really have any control over the underlying WebKit
iOS browser apps are required to use WebKit, but you can pretty heavily modify things. There is a Tor browser app available, it runs Privoxy internally and proxies all traffic from that WebKit instance. Just one since I'm guessing nobody else could be bothered?
Edit: you also can't control tracking to the same extent the forked Firefox that Tor uses can.
This might be an over simplification, but my understanding was that all browsers on iOS were essentially just reskinned safari. And that is an app store restriction.