The apps literally just tell the OS to call a certain number AND provide a menu for the call, latter chosen via the default apps system. So if you have dialer X selected but dial a number using Y, it will just open X's menu.

You can just not have a phone app selected (AOSP GSI builds come this way for some reason). The behavior for that is that the phone will vibrate for a call but you can't accept it. If you then dial a number using the app that's not actually selected, it will call the person but the dial pad will never close and there will be no way to hang up. Source: truss me bro (or more seriously, it was from a YouTube video).

Point is, the apps do nothing for the call. The audio doesn't even go through them.

There used to be call recording dialer apps, but what they did is literally tell Android to record a video, since regular audio recording isn't allowed during a call. It wants a file path but some Unix magic makes it feed to a file converter instead. The file converter then spits out an audio file. Dead after Android 9.