So re. #25thAmendmentNow here's how it would work:

Vice-President Pence and "a majority of either the principal officers of the executive departments"* would sign & send a letter to Congress saying Trump is unable to discharge duties of Presidency. 1/

That letter makes Pence into the Acting President.

But if Trump then sends his own letter to Congress saying that he is able to discharge the duties of the office, he resumes the Presidency.

Trump resumes the Presidency UNLESS Pence and the cabinet* (within 4 days) send their own letter to Congress saying that Trump is still unable to discharge the duties of the office.

Pence would then remain Acting President and Congress would have up to 21 days to vote on removing Trump from office (a two-thirds vote by both chambers required to remove him)**

* "principal officers of the executive departments" means Cabinet Secretary. However, we don't really know whether an "Acting" Secretary counts. https://www.washingtonpost.com/outlook/2019/07/22/trump-has-lot-temps-top-jobs-would-they-get-say-removing-him-his/

** Of course, Donald Trump's term of office would expire before the 25th Amendment requires Congress hold a vote on removing him from power.