The fabric for the clothes Zhang Family had made for him was purchased from Xiangji Fabric Shop. They bought too much and didn't use it all, so they used the leftover to make him a pair of underpants.
Now that he saw a pair of underpants, identical to his, in the hands of the Sun family, most certainly, the fabric for these came from the Xiangji Fabric Shop as well.
There are only two fabric shops in Bailu Town. One sells rough linen clothes for the common folk, and the other, Xiangji Fabric Shop, sells only fine silk and satin.
The underpants in the hands of the Sun family looked new. Therefore, there was a good chance that she went to the town herself to buy the fabric.
However, he was curious as to how she found out that he possessed a pair of underpants like these.