It’s a modern Inauguration Day tradition for outgoing presidents to write a letter to their successors and leave it on the Resolute Desk in the Oval Office.
Barack Obama wrote to Donald Trump, in part, “Regardless of the push and pull of daily politics, it's up to us to leave those instruments of our democracy at least as strong as we found them."
George H.W. Bush wrote to Bill Clinton, “I wish you well. I wish your family well. Your success now is our country's success. I am rooting hard for you."
Attached is what Donald Trump left for Joe Biden.