The "15021" tag is likely an internal tracking number from a specific IPA repository. Users are searching for it because they found a link in a Discord server, a Reddit thread, or a Telegram channel promising that this specific file works perfectly with their current iOS version.
: Listen to music or podcasts while using other apps or even when your screen is locked. youtube 15021 ipa download exclusive
Are you interested in the (like ad-blocking or downloading) included in that build? What is an IPA file? - BrowserStack The "15021" tag is likely an internal tracking