United States-Mexico-Canada Agreement Foreign-Trade Zone Modernization Act of 2021 or the USMCA FTZ Modernization Act of 2021 This bill requires the International Trade Commission to investigate tariff policies related to foreign-trade zones (FTZs), which are designated sites at which special customs procedures may be used to allow for domestic activity involving foreign items prior to formal customs entry. The investigation must examine (1) differences in tariff treatment by the United States, Canada, and Mexico; (2) any effects of such differences on the cost-competitiveness of U.S.-manufactured products compared to products manufactured in Canada or Mexico; and (3) how FTZs in the United States could be better employed to redress and mitigate any inequities.