How can I record internal audio? When I try use recording apps to record audio that comes from another app they don't work - actually the recording doesn't start at all. so how can i record this audio output?

I am looking for a solution for android 8.
as far as I know, Google policy don't allow internal audio recording on Android.

But here are some tips:

Some phones have the feature within their UI like MIUI or EMUI or samsung. But you may not hear the sounds while internal audio is recorded.
Also, you can try Elgato game capture card. Many other apps are available (but they frequently need Root though.)

Use audio splitter. You may connect one to headphones to hear what you are recording. Connect other to PC through aux cable and record in windows. Use any screen recorder to record screen and finally mix the audio and video file.