UI wrappers around the exact same WebKit used by Safari, because Apple doesn’t allow third-party browser engines in the App Store, and they don’t allow mass distribution of non-App Store apps on iOS.Chrome? Edge?
For years, third-party browser apps actually had to use a limited version of WebKit that lacked some performance improvements Apple had brought to Safari. Eventually, Apple relented and made the same version of WebKit that Safari uses available as a public API.