La donna è mobile - Motive A
qual piuma al vento - Motive A but sequence at a lower pitch
muta d'accento - Take the "mobile" part of Motive A but elongate it to create Motive B
e di pensiero - Motive B sequence at a lower pitch
"Sempre un amabile
in pianto o in riso
è menzognero" this is a melodic repeat of the first four lines (ahh Giuseppe Verdi you were lazy there )
La donna è mobil - Motive A but the leap from "e mobile" is higher to form Motive C
qual piuma al vento - Motive C but sequence higher pitch
muta d'accento - Motive C however the middle note cent - is suspended to a longer duration
e di pensier - Motive B
e di pensier - Motive B repeat
e di pensier = elaboration of Motive C
So although most of the melody line are different to each other but they are all related to the opening motive.
Development of "vocal tunes" that is instantly memorable was due to the development of manipulating motive. Thsi is essnetially now part of the furniture of all vocal based music including pop music
Now pop songwriters do not consciousness do this because this techniques is already ingrained in the cultural background and they will unconsciousness do this (similar how people who do not have formal musical training and making up music on the spot will automatically sing in major or minor scale even if they know nothing about this).
This is what I meant by melody that is flowing but at the same time has a sense of unity.