YouTube music playlists use multiple signals to gather music from across the YouTube platform. Machine learning, community signals, signals from other Google goods and services, and manual human inputs are some of the examples of these (including the aggregate listening pattern of a user).
These playlists are one of the several sources of information for YouTube’s recommendation algorithm. The better a song performs in its curated playlists in terms of engagement, the likelier it is to be discovered by users.