When you ask what technique Billy Joel uses to "establish unity between the lines", I assume you're talking about "We Didn't Start the Fire"? I assume this because it's one of the most infamously deconstructed songs in both History and English classes.
Billy Joel uses two major literary tactics in this song that establish unity and make it so catchy and memorable. First of all, Joel plays off the beat of the song by singing the lyrics in a linear fashion (like a shopping list). Notice the music itself doesn't change between the chorus and the rest of the song.
Secondly, and more importantly, he take advantage of the allusion in his lyrics. Billy Joel's "We Didn't Start the Fire" is full of references to real events and icons of history. An allusion is literally just a reference to something that you could just understand.
ex: "He was more popular than Jesus".