"Some other body" is quoted from the 25th Amendment, which empowers Congress not to do the same thing that the Vice President and Cabinet can do but to create "some other body" that can do what the Vice President and Cabinet can do. They could pass a law that empowers Congress to do that, but that would be less likely to pass than passing a law that empowers some committee because Congress has other things to do.

Impeachment action is limited to "Treason, Bribery, or other high Crimes and Misdemeanors".

Impeachment is not permitted for simple incompetence.

Congress is only empowered to remove the president by convicting him of a criminal act. Congress does not have the power to remove the president for dementia. Dementia is not a "high crime" or "misdemeanor".

To remove the president for dementia (or simple incompetence or other non-criminal disqualification) requires the 25th amendment. The VP needs the support of either the majority of the cabinet, or a majority of "some other body" that Congress has established for the purpose.