Section 4. Whenever the Vice President and a majority of either the principal officers of the executive departments [the Cabinet] 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.
Not true. In addition to the Cabinet, the 25th Amendment also provides for Congress to appoint a committee to do this in cooperation with the Vice President, and this process was started by Rep. Jamie Raskin on April 14.
Here’s the actual text of the relevant section of the 25th from Wikipedia: