Relative passages of 25th Amendment
"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."
TL/dr: It takes the VP and a majority of either the cabinet or one of the Houses of Congress to invoke the 25th Amendment and transfer the powers of the presidency to the VP.
I find it interesting (dons tinfoil hat) that Nancy wants to talk 25th Amendment AND Pence rushes off to be back in DC. Of course, it's all going to turn out to be a nothing-burger but if it gets under Scrotus' skin I'm good with it.