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This app is always free and basically allows you to choose YouTube songs and play them in the background aka turn off your screen and listen. Also has the option of only playing audio at low or original quality to save data. Won't play every song but will play about 90%
No need for this, just use YouTube Vanced like everyone else. Works as a YouTube client, but removes all ads and has background play. Works 100% of the time and uses the modern YouTube client.