The Twenty-fifth Amendment (Amendment XXV) to the United States Constitution says that if the President becomes unable to do his job do to mental or physical issues the Vice President becomes the President. This can happen for just a little while, if the President is just sick or disabled for a short time. This link goes into detail
https://constitutioncenter.org/interactive-constitution/amendment/amendment-xxv.
Impeachment is when a POTUS commits crimes or uses the office for illegal activities. There's more to it than this, you will have to look it up to get details. The House does the investigation and provides the evidence to the Senate. The senate holds the trial based on the evidence the house provides.
https://constitutioncenter.org/interactive-constitution/amendment/amendment-xxv.
Impeachment is when a POTUS commits crimes or uses the office for illegal activities. There's more to it than this, you will have to look it up to get details. The House does the investigation and provides the evidence to the Senate. The senate holds the trial based on the evidence the house provides.