Casa Grande Ruins National Monument Boundary Modification Act of 2021 This bill modifies the boundary of the Casa Grande Ruins National Monument in Arizona and provides for a study of the feasibility of designating the Don Young Legacy Trail in Alaska. Specifically, with respect to the monument, the bill authorizes the Department of the Interior to acquire private or state land or interest in land by donation, exchange, or purchase from willing sellers in order to expand the boundaries of the Casa Grande Ruins National Monument in Arizona. The bill transfers administrative jurisdiction over specified parcels of Bureau of Land Management land to the National Park Service (NPS) or the Bureau of Indian Affairs (BIA) and specifies an exchange of land between the BIA and the NPS. Upon the acquisition of land or an interest in private or state land, and with respect to any transferred land, Interior shall administer the acquired or transferred land as part of the monument, and modify the boundary of the monument to include the acquired or transferred land. Interior may enter into an agreement with Arizona to provide for cooperative management of approximately 200 acres of specified state lands.