Have you ever wondered how different hand tied wefts look to the machine weft? It mainly has to do with the top "seam" where all the hair is sewn in. The hand tied weft is done by hand and is super delicate. It is what breaks down on hair extensions (other than super blonde ends) first. The weight of each weft is measured in grams. A hand tied weft generally is 12-15 grams of weight (also depends on the length).
The machine weft is is hardier, but some finer haired folks don't have the hair density to hide it when it's just under their hair. The video shows how thin the hand tied is versus the machine weft. They can be combined.