More:
- you can't uninstall Safari. The DMA clearly requires Apple to allow uninstalling it.
- the Share Sheet for in-app webview privileges Safari (Safari-specific "Add to Reading List", "Add Bookmark" at the top) regardless of default browser.
- apps with in-app webviews can roll out a custom engine, but otherwise it uses WebKit regardless of default browser. It should obviously use the webview facility provided by the default browser, if it offers one.
- confusing to change defaults. There's no central "Default Apps" screen in Settings. You have to go in the Settings app, scroll way down to a given browser, and then you can change the default. So the default browser setting is weirdly treated as an app-specific setting rather than a system setting.
- If Safari is not the default, you can change your default browser within "Settings.app >> Safari". If Safari is the default, you can't change your default browser there at all. Whereas the default browser selection menu will always be shown in the Settings.app submenu of third-party browsers.