From the Introduction: "At its heart, a presidential transition is the transfer of executive power from the incumbent President to his successor. A single step--taking the oath of the office of President--accomplishes this transfer. Yet the transition process is more complex, beginning with pre-election planning and continuing through Inauguration Day on January 20. It involves key personnel from the outgoing and incoming Presidents' staffs; requires resources; and includes many activities, such as vetting candidates for positions in the new Administration, helping to familiarize the incoming Administration with the operations of the executive branch, and developing a comprehensive policy platform. The Presidential Transition Act (PTA) of 1963, as amended, established formal mechanisms to facilitate presidential transitions and authorized the Administrator of General Services to provide facilities and services to eligible presidential candidates and the President-elect."
CRS Insight, IN11538
Congressional Research Service: https://crsreports.congress.gov/