Vocal training with a couple of personal instructors and several books, CDs, cassettes. Best of which was seth riggs "singing for the stars"
Songs on spotify or apple or CDs represent more than an octave or two of vocal range.
It is almost all within my vocal range but I'm not a classical singer and some part s will sound better than others.
And some pieces might present obsticles.
In my own voice, a strong voice might be better represented by a falsetto than a high tenor.
If singing extremely low or high having reference notes might be to an advantage.
I believe music in film often requires a greater level of careful consideration than some film directors are willing to donate thier time to.