If Biden + Harris win on Nov. 3. Harris will leave her elected position and be replaced by Obama. Biden will subsequently leave his elected position and Obama will be inaugurated as president on 1/20/21 under the 22nd and 25th amendments.
The 22nd amendment states that no person shall be “elected” to the office of the presidency more than twice. Since Obama was not “elected” to the presidency this time around the supreme court will rule that because of the way the amendment was originally published (“elected”) that he can be president.