25TH Ammendment, Section 4:

Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments or of such other body as Congress may by law provide, transmit to the President pro tempore of the Senate and the Speaker of the House of Representatives their written declaration that the President is unable to discharge the powers and duties of his office, the Vice President shall immediately assume the powers and duties of the office as Acting President.

The "principal officers of the executive department[s]" are the fifteen Cabinet members.

Secretary of State

Secretary of the Treasury

Secretary of Defense

Attorney General

Secretary of the Interior

Secretary of Agriculture

Secretary of Commerce

Secretary of Labor

Secretary of Health and Human Services

Secretary of Housing and Urban Development

Secretary of Transportation

Secretary of Energy

Secretary of Education

Secretary of Veterans Affairs

Secretary of Homeland Security

What if Joe Biden pre-emptively FIRES any disloyal among those 15 cabinet members? Then they can't sign a declaration or have a majority. 😊