Where would a US President serve his time if he was convicted of a crime?

Even if Trump was solidly convicted......the most that would happen is that he is driven from office.

The VP becomes President, and the first thing he would do, is PARDON the President, to avoid the disgracing spectacle of an ex- US President sitting in a jail cell.

Just as Ford immediately pardoned Nixon. ......and Gore would have done the same if Clinton was removed from office after his impeachment trial.

But Clinton was not removed, so Gore never got his chance to pardon him-

A Russian Gulag

One of the federal prisons I assume. But he would be a 'former' president at that point.

probably in jail just like everyone else

5th Avenue in Manhattan, last time I checked.

A Federal penitentiary.

In his new big Black husband Bubba's cell.

in a supermax facility

A sitting president cannot be indicted for a crime. He can be impeached, removed from office, and then tried in the appropriate court. He would then go to state or federal prison, depending on the jurisdiction.

No where. No U.S. President would ever be sent to prison. The President who takes over would immediately 'pardon' them. Just as President Ford 'pardoned' President Nixon.

Federal prison