The natural voice and the falsetto

Do you know what the difference is between a natural voice and a falsetto?
This is very complex to explain and there is no simplified definition, because the usage the muscles are different and depend on the individual.
There are two soft tissues that are truly relevant to the natural and falsetto voice.
(1) Vocal cords’ muscle
The vocal cords’ muscles exist inside the lamina of the thyroid cartilage and the cricothyroid ligament, through the front of the vocal ligament, and the vocal process.
Together the vocal ligament and the arytenoid cartilage control the tension of the vocal muscles, by rotating and sliding, up and down the vocal scale.
The vocal scale and pitches are changed by extending and contracting the muscles. Vocal cord muscles cannot extend by themselves.
(2) Vocal cords’ mucosa
The vocal cords’ mucosa is (generally thick) membranous tissue.
Although the vocal cords’ mucosa (density) is controlled by the recurrent laryngeal nerve. The vocal cords’ mucosa does change when vibrating and (the density) depends on that status.
Therefore, the pitch is changed by thinning or thickening the mucosa. That is why, the voice will sound hoarse if it is dry.

The vocal cord when opening

The green is the vocal fold and the blue is the edge of vocal cord

The vocal cord when closing

The green is the vocal cord muscles and the blue is the vocal cords mucosa

Natural voice:The status when the vocal cord muscles and the vocal cord mucosa vibrate.
Falsetto voice:The status when the vocal cord muscles harden, stop vibrating and only the edge of the vocal cord mucosa vibrates.
A mixed voice, refers to mixing the above two voices properly. You cannot acquire the proper mixed voice if your vocal cord muscles are not flexible and your vocal mucosa is not present and well hydrated.
For getting flexibility of the vocal cord muscles, you have to improve the circulatory function of the intralaryngeal muscle and move the arytenoid cartilage.

Note1:The red line is the vocal cord.

Note2:The blue circle is the vocal cord.