Do plain weave fabrics must be all cotton?

Plain fabrics are not necessarily cotton.
Plain weave is a textile tissue form, and its counterpart is twill weave. A fabric formed by interweaving warp and weft yarns one up and one down. The numerator represents the warp weave point, and the denominator represents the weft weave point. The plain weave is only related to the thickness of the fabric and has nothing to do with whether it is pure cotton. Plain weave fabrics are thinner, and twill weave fabrics are thicker.