The US Constitution does not list who becomes president if the president and vice president are not able to serve. It does, however, provide that the Congress can create a statute to do that.

Congress did. Presidential succession goes like this:

President - Biden is a threat to national security due to his financial ties to China, which just interfered with critical infrastructure of the United States, and therefore became an enemy of the USA. He cannot be president. He might have also been directly engaged in election tampering. Let's see what the evidence shows.

Vice President - Harris is not constitutionally eligible to be vice president because she is not a natural born citizen. In addition, as a senator, she aided China by voting to certify a fraudulent election.

Speaker of the House - Pelosi also voted to certify a fraudulent election.

President Pro Tem of the Senate - Grassley also voted to certify a fraudulent election.

Secretary of State - Pompeo might be the one to run the interim government until things get sorted out.

Secretary of Treasury - Steve Mnuchin - If Pompeo resigns or chooses not to accept the role, it would fall to Mnuchin.

Secretary of Defense - Christopher Miller - If both Pompeo and Mnuchin step aside, then Miller would be handling the interim period.