Complex sentence: Ono gave them four demo songs for possible inclusion, of which "Free as a Bird" and "Real Love" were used to record overdubs on, and have become the final new recordings issued as The Beatles.
Split into simpler sentences: Ono gave them four demo songs for possible inclusion, of which "Free as a Bird" and "Real Love" were used to record overdubs on. Subsequently those two songs have become the final new recordings issued as The Beatles.