This is a hypothetical. Let's say Trump's cabinet has not resigned yet. Joe Biden is sworn in and, overwhelmed by the emotions, he has a stroke and can't move or speak, or he is in a coma, but he is alive.
The cabinet does not want to hand over the power to Harris and so they turn down the vote to trigger the 25th Amendment. Who is in charge? Who makes executive decisions? No one but the President can fire principal officers. So they cannot be fired, but they can be impeached. Let's say the impeachment fails in the Senate because the Senate does not want to give power to Harris, either.