In a nutshell...

From a text I sent to a student, which by dumb luck is about as concisely as I’ve ever described excellent, sustainable singing:

“Sometimes when I speak or sing, I feel like the sound is originating at the vocal folds, rather than starting with the breath and then merely passing through the vocal folds into the resonant space (mouth). When I focus on the activity at the ribcage and in the vowel space and “ignore” the throat, I get my best results.”