Im currently working with the xposed framework on Android 8.0. How is it possible to find all the method names of an app to hook? For example, i want to hook the method that is called if i add a new contact. Would it be possible to reverse engineer the apk to insert an logcat output into all methods, that shows which method was called?
There a various methods for identifying the relevant methods.
The first and most important one is knowledge of the official Android API. All Java/Kotlin based apps at some point use the classes and methods defined i the API. And the most important fact is that because the Android API belongs to Android and not to the app it can not be obfuscated.
Second you can reverse engineer the app itself using tools like Jadx, apktool, Ghidra, JEB... Just make sure that the tool you use does not rename the class names (e.g. to avoid name collisions or make obfuscated class and method names better readable) or at least allows you always show you the original class and method name. Because trying to hook a method by it's name will fail if you use a name generated by the APK reversing tool.
Also a very helpful tool that allows to identify a lot o the internal on a running program is frida-trace. As long the the executed app has no anti-debugging or anti-frida measures in place you can attach frida-trace at any time to an app on a rooted device and create execution traces you can later use to hook the methods using xposed or directly using frida.