You can find them at - Internal Storage/Android/data/com.amazon.mp3/files/Music/Purchased but this has to be done on a PC you won't actually have song files they are encrypted so they can only be used with the Amazon app.
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If you have purchased Amazon music files specifically for download, the proper pathway should be Internal storage/Android/data/com.amazon.mp3/files/Music/Purchased
If you just have an Amazon music subscription, downloads for offline listening will be contained within the Amazon music app and are inaccessible through the regular file system. This is a security measure designed to prevent piracy.
