If we stick with Garcia’s definition of “register” as tones following a similar mechanical principle, then we have the chest (thyroarytenoid dominant) and the falsetto (cricothyroid-dominant), and that middle area that gets all kinds of names is where they dance. Sometimes one leads, sometimes the other does. Each can go to the other’s neighborhoods, but they should take the native along as an escort.
People who do not have a smooth register coordination in the passaggio can learn to hide it with various mechanical and/or resonance strategies, the depressed larynx being one of the ones most used in classical singing. In other styles, or depending on the personality, singers sometimes just let abrupt transitions be part of their singing. But those who have a smooth middle to their voices are dealing with coordinating two main muscular subsystems in the voice, not creating new muscle systems. I try to be very careful to use the word “middle” by itself or with the word “range” and never with the words “voice” or “register” because that would mess up the idea of registration as I try to convey it. “Middle register” or “Mix register” don’t make sense to me. “Mixing” is a thing we DO with our two registers, not a container that we pour the voice into. “Middle” is the PLACE where it usually happens.