Many apps let u use only the default built-in camera app. They not allow you to choose a third party app, such as the app "fake camera". Is there a way to remove/replace the default camera app? Any ideas?
Yes, it's possible. At least, on some devices. But without knowing anything about your device, it's hard to answer your question.
For example, on Android 8, you have a menu like this: If you have a search option in your settings, then you can easily search for "default", and you will find it easily.