Two pieces, not a saree
The mekhela is the lower piece, worn wrapped and pleated at the waist. The chador is the upper piece, draped over the shoulder and tucked. They are woven and sold as a set.
It is not a saree and it is not draped like one. A saree is a single length of cloth. Anyone who tells you a mekhela chador is “the Assamese saree” is giving you a useful shorthand and an incorrect one.
A riha is a third, narrower piece worn with the set on the most formal occasions, particularly at weddings. A riha-mekhela set is not the same thing as a mekhela chador, and the difference matters if you are buying for a wedding.
Pat, not muga
This one is pat: Bombyx mori silk, the fine glossy white or off-white thread. Pat is what most formal mekhela chador is woven in, and it takes colour and zari beautifully.
Muga, the golden one, is more expensive and is what you see at the highest-formality occasions. A genuine muga mekhela chador costs a multiple of this. If you are offered one cheaply, it is blended, and how to tell real muga from blended explains what to look for.
Handwoven, and what that means for the cloth
Slight irregularity across the width, a motif that is not machine-identical repeat to repeat, and a selvedge with a little variation. Those are the marks of a loom with a person at it.
Full explanation of the garment, how it is worn, and what you should expect to pay at different qualities: the mekhela chador guide.
