Everything is still Safari underneath. That’s probably never changing as long as Apple has a say (that new EU law may force it).

But this would mean you could use ApplePay from the current FireFox app.

Letting all browsers with a Safari engine use all the features of Safari-Safari fed be a way to nudge users to keep using Safari browsers instead of other browser engines, when that EU law forces Apple to open up the platform. The other browsers could be forever behind is iOS features. However that's going to make the browser experience closer to native.

I'm glad Apple is opening up features for other browsers to use on iOS, but I don't understand how this nudges users? As soon as it is permitted, Firefox, Chrome, etc will switch from their WebKit based implementations to using their own engines.

I might be wrong but if the upgrade from Chrome-Safari to Chrome-Chrome means that users lose Apple Pay inside the browser, something they just got used to have, maybe they switch to Safari-Safari to keep the feature.

You mean everything is WebKit underneath. Safari and Chrome on iOS are very different browsers.

No, I believe they meant Safari underneath, which is often used as shorthand for the specific rendering engine based on WebKit that powers Safari on iOS and comes with its own quirks. Chrome does not use the same engine on any other platform as it has deviated significantly.

The section on “Apple’s Open Source Claim” in this blog post [1] goes into a lot more detail on the relationship between Safari, WebKit, and the Chrome/Chromium rendering engine.

[1] https://infrequently.org/2021/08/webkit-ios-deep-dive/