Why are my mp3 files not in my music folder?

I have a lot of Mp3 files on my phone (Huawei Mate 20), but unfortunately not all files are in the Music Player app. I can only find the mp3 files in the file manager. I can open this from the file manager but the audios are not to be found in the music app. Why is that?

Maybe wrong file type?

No, they are all mp3 files

The music app is original from Huawei, if you don't download Samsung music, it works better