National Defense Authorization Act for Fiscal Year 2021 This bill authorizes FY2021 appropriations and sets forth policies for Department of Defense (DOD) programs and activities, including military personnel strengths. It does not provide budget authority, which is provided in subsequent appropriations legislation. The bill authorizes appropriations to DOD for Procurement, including aircraft, weapons and tracked combat vehicles, shipbuilding and conversion, and missiles; Research, Development, Test, and Evaluation; Operation and Maintenance; Working Capital Funds; Chemical Agents and Munitions Destruction; Drug Interdiction and Counter-Drug Activities; the Defense Inspector General; the Defense Health Program; the Armed Forces Retirement Home; Overseas Contingency Operations; and Military Construction. The bill authorizes the FY2021 personnel strengths for active duty and reserve forces and sets forth policies, including those regarding military personnel; acquisition policy and management; transfer of property for law enforcement activities; financial oversight; Space Force matters; artificial intelligence; advanced microelectronics; international programs; nuclear security and energy; National Guard and Reserve Forces facilities; compensation and other personnel benefits; health care; DOD organization and management; civilian personnel matters; diversity and inclusion; matters relating to foreign nations; hostage recovery; emergency alert systems; the autonomy of Hong Kong; and strategic programs, cyber, and intelligence matters. The bill authorizes appropriations for base realignment and closure activities, and maritime matters. The bill authorizes various intelligence-related activities and programs for FY2021. The bill also authorizes appropriations and sets forth policies for Department of Energy national security programs, including the National Nuclear Security Administration and the Defense Nuclear Facilities Safety Board.