21st Century Buy American Act This bill modifies the requirements for federal agencies to grant exceptions and waivers to Buy American requirements. The bill also (1) requires the General Services Administration to establish a public website with details regarding waivers and exceptions that have been requested, are under consideration, or have been granted by agencies; (2) increases the domestic content percentage requirement to 60%; and (3) authorizes the Department of Defense to establish a program to make or guarantee loans under the Defense Production Act to certain domestic manufacturers.