I had deleted several files like from my Nexus 6P, I want to permanently erase all deleted files, how to do it? where are the deleted files stored?
They are not 'stored' anywhere. The system marks them to be overwritten, so whenever that disk space is needed, it will write to it. There is no specific location,nor a way to trigger the overwrite. It'll happen as your device is used
Short of professional recovery tools, there is no way to get these files back especially as Android by default encrypted your phone
If you are really concerned, there are PC apps that can military grade format your phone