In the Genesis Ark story, Shem is traditionally regarded as an ancestor of the Israelites.
Shem is one of Noah’s three sons, alongside Ham and Japheth. In Genesis 10, the descendants of Shem are listed in the “Table of Nations,” and Genesis 11 traces a line from Shem through several generations to Abram, later called Abraham. Israelite ancestry is traditionally connected with Abraham’s family line.
The term “Semitic” is derived from Shem’s name, although modern linguistic classification is not identical to a simple biblical genealogy. Semitic languages include Hebrew, Arabic, Aramaic, and several ancient languages, so the term covers a broad linguistic family.
Ham and Japheth also have important roles in the biblical genealogy. Genesis associates different peoples with each of Noah’s sons, but the line leading toward Abraham runs through Shem. Lamech, by contrast, is Noah’s father, not one of Noah’s sons.
Thus the answer depends on the genealogy that follows the flood narrative, not on which son built or commanded the Ark.