I don't expect many EU apps to bundle Chrome because they can't use it outside the EU and the benefits of bundling a different browser engine for in-app browsing would be limited for the types of basic browsing that is typically done inside a non-browser app. Additionally, Apple is imposing additional requirements on apps that embed alternative browser engines, including requiring apps to be updated within 15 business days of a browser engine update and requiring the app to only be available in the EU.

As a result, I would expect Chrome/Firefox/Edge/Opera to ship their own browser engines on iOS but am doubtful that many other apps will embed Chromium/Gecko with the possible exception of apps that only operate in the EU.

You don't need custom versions for Chrome for tracking. Plenty of apps force links to open in-app, inject JS and then you have to make a few additional taps to get it to open in untampered Safari

Apples rules make it clear you can't bundle Chrome. Your alternate browser has to be fundamentally a browser, not something else that happens to have a browser as an extra feature

This doesn't happen on Android really. Most developers use the built in WebView