I've captured Z creating one of her songs on the piano. Sometimes her creations are difficult to play with both hands, so she'll play one hand and record it, and then accompany it with her other hand. That's what she did here.
Z creates many songs - some songs are completely her own and some are songs she's figured out (Christmas songs, popular tunes, tv shows...). When she figures out other songs, she often creates her own chords and accompaniment to go with it.