History, Evolution, and Practices of the President's State of the Union Address: Frequently Asked Questions [Updated April 16, 2021] [open pdf - 953KB]
From the Summary: "The State of the Union address is a communication from the President to Congress in which the chief executive reports on the current condition of the United States and provides policy proposals for the upcoming legislative year. The address originates in the Constitution (Article II, Section 3, clause 1), which requires that the President 'shall from time to time give to the Congress Information of the State of the Union, and recommend to their Consideration such Measures as he shall judge necessary and expedient.' [...] This report takes the format of answers to frequently asked questions about the State of the Union address."
CRS Report for Congress, R44770
Congressional Research Service: https://crsreports.congress.gov/