Because your phone may contain all manner of audio and video files scattered around its storage locations which you don't want to see in Poweramp's curated music library, the first thing you need to do after installing the app is to narrow down what areas it will scan to find your music collection. This is done via PA Settings=>Library=>Music Folders, but before it starts scanning you will need Android to allow access.
First, it's worth noting that Google does not make this process as easy as it should be. It used to just work out-of-the-box - file and folder selection was as easily as accessing your files on a Windows PC. Then after a few years they introduced a permission-based system, although all you needed to do was click one button to confirm that you agree for Poweramp to be able to access media files from your device's storage or SD Card.
This method is usually still possible, even up to Android 12, if you enable PA Settings=>Misc=>File Access Legacy Mode and then accept the confirmation prompt:
Once you have done that, you can use PA Settings=>Library=>Music Folders to select (tick) any locations that you wish Poweramp to scan for music, either by choosing the whole of a storage location or just certain folders within it (the latter more-specific solution is generally preferable, to avoid accidental clutter).
However there is no guarantee that this method will continue to work as Google continues to clamp down on access rights which it does not wish you to have, which brings us to the more modern solution:
Storage Access Framework (from Android 10 onward)
This is now Google's preferred method for apps to gain the permissions that they need to read/write files. It is much more restrictive than the old legacy method, and also considerably slower in some instances. However it should continue to work with all future release of Android.
You are not meant to select whole root storage areas with SAF (such as choosing to use all of an SD Card) but must instead request the specific folder to which you require access, which will also provide access to any of its subfolders. Some system folders (most notably Download on internal storage) are, or will be, prohibited and should be avoided.
The process is similar to the legacy method, but you need to use Android's folder-selection system to grant access first, and then you may need to use Poweramp's Music Folders system to refine that choice. Here is the procedure laid out in detail, step-by-step:
Go to PA Settings=>Library and select Music Folders:
Either tap 'Enable' next to one of the storage locations, or 'Add Folder or Storage':
The next screens are Android's, not Poweramp's. If necessary, use the three-lines menu icon in the top-left to choose where your desired music folder is located (e.g. internal memory or SD Card) and then drill down into that folder (e.g. 'Audio')and once you've found the location that you want, tap 'Use This Folder' and finally confirm your choice:
Once you have granted access, you should see your chosen folder in Poweramp's Music Folders screen. You can drill down to tick or untick any subfolders if you wish, or by default just leave the whole folder selected:
When you exit from that Music Folders screen, Poweramp will start to scan the new location looking for music files, which might take a while:
If the new content is not visible in the Library after adding a new drive or folder, try using the Full Rescan option in PA Settings=>Library.
Note: if you see an error message when you are trying to use the Grant Access system, it may be because the version of Android on your device is missing some critical system components, such as the Google 'Files' access module. See the following FAQ for help:
You may wish to use some of the extra options in PA Settings=>Library to avoid discovering very short audio files (such as notification tone sounds) and also disable scanning for Video files (for which only the audio content can be played anyway):