The phrase "half airplane, half auto" is sung to a melody something like this:
"Half airplane" is sung to notes that total five beats, and "half auto" is sung to notes that total six beats, so it's not quite exact, but there is a sort of musical balance that matches this "half" and "half."
This balance is also present in the lyrics alone: the two phrases have the same number of syllables, and there's a parallelism in the initial letters of the words.
