It’s a double edge sword, I’d would be nice to be built directly into Android AOSP, but at the same time if it was, phones without the latest Android wouldn’t even have it

There’s ARM generic that some distros support, but depend on your CPU supporting UEFI

There’s two in-app browsers experiences in Android. Developers can either make a pretty facade to a WebView like Facebook or Instagram, that one you have the capabilities to inject JS or CSS as needed, like here. The other one is Chrome Custom Tabs, which browsers like Firefox and Chrome support, developers don’t have access to your data in here