The Appalachian Trail Centennial Act establishes a framework for cooperative management of national historic and scenic trails, designating the Appalachian Trail Conservancy as the primary operational partner for the Appalachian National Scenic Trail and authorizing the federal government to designate other eligible nonprofit organizations as operational partners for other trails. The bill clarifies the division of roles between federal administration, land management, and trail operations, empowering volunteers and nonprofit organizations to undertake non-governmental functions including maintenance, facility construction, and resource conservation. It requires comprehensive management plans and establishes mechanisms for cooperative agreements between federal agencies, state and tribal governments, nonprofits, and volunteer organizations to manage and operate these trails collaboratively.