By Inauguration Day, it was clear: She was going to keep her day job.

“Of course, you know, people around me said, ‘No, no, you can never do it,’” Dr. Biden recalled, reflecting on the process in a later interview.

“And I said, ‘I’m going to do it. So figure it out.’”

‘Something bad happened’

On Jan. 6, 2021, Donald J. Trump wanted to stay in the White House.

But his wife just wanted to go home.

For months, Mrs. Trump had taken to walking around the Executive Residence in hotel-style terry cloth robes. Throughout her husband’s presidency, she often perched on the bed in his room to listen to or join in on his calls with advisers and allies, Stephanie Grisham, Mrs. Trump’s former press secretary, said in an interview.

Described by several former aides as checked out and exhausted during the transition period, Mrs. Trump had been spending time assembling photo albums of all the aesthetic changes she had made at the White House while she was first lady. (“All she cared about was those photo albums,” Ms. Grisham said, using an expletive to describe the albums.) Mrs. Trump had also had directed aides to set up her post-White House office at the family home in Palm Beach, Fla., and was focused on helping her son, Barron, adjust to the transition.

According to several former aides, Mrs. Trump had visited the East Wing, where the first lady has an official office and staff, so infrequently that her empty office had been converted into a gift-wrapping room. The White House Military Office, sensing an opportunity to claim coveted White House territory, converted part of the unused East Wing into a secure communications facility.