21st Century Buy American Act This bill modifies domestic product preferences for federal acquisitions. The bill also allows the Department of Defense to make or guarantee loans to manufacturers under the Defense Production Act for specified uses, such as to increase the capacity to produce items that are vital to national security. The bill requires more than 60% of a product's cost to be from domestic components for the product to qualify as American for purposes of the Buy American Act. The bill narrows the circumstances under which the overseas use exception and the public interest exception to domestic content requirements may be made. The General Services Administration must maintain BuyAmerican.gov, which must include and make available to the public (1) information on all waivers and exceptions to domestic product preference laws requested, under consideration, or granted; and (2) publicly available contact information for the contracting agencies. No requested waiver of a domestic product preference law may be granted if (1) the request was not made available to the public, (2) the information available to the agency concerning the request was not made available to the public, or (3) no opportunity for public comment concerning the request was granted.