Voice teachers, typically but not always, refer to vocal registers. In the “normal” vocal register (modal) your larynx is positioned toward the back of your throat. In the so called falsetto register, the larynx is positioned toward the front.
The notion of “registers” is based on the idea that our voice breaks. It is a tautology. We say that there are vocal registers because our voice breaks. We say that our voice breaks because there are vocal registers.
If you attempt to entone a pitch that is too high, when the larynx is positioned in the back, your voice breaks. Similarly, your voice breaks if the larynx is too far forward and you try to entone a pitch that is too low.
In between falsetto and modal is the area called the passagio. Again, based on the tautology that we have vocal registers. You can entone a pitch within the passagio if the larynx is neither too far forward nor too far back.
In other words, there are no vocal registers, in order to not break you need to position your larynx correctly.
A way (not the one and only way) to avoid the first break, when singing a siren (sweeping from high to low or low to high) is to make your throat form the shape that it does when you yawn, when you reach the place where your voice will break.
Here is a clip of me doing this with the yawn tone clearly audible:
With a slight change, the voice sounds less like a yawn when in the area where my voice usually breaks:
The slight change was to not open my throat as much.
The difference in tone is more obvious when I try to speak in that area currently. That is what I am working on. But, it appears that I should be able to have essentially the same control through out the range that was thought to span falsetto and modal. Probably, the same is true for the so called whistle and bass registers.