Re sitting in the courtroom, I believe that a defendant must be in the courtroom whenever proceedings are taking place. But once the jury is in the jury room and there is nothing 'going on', they can be 'elsewhere' but must be within 'calling distance' in case of a verdict or further proceedings.
I seem to remember Trump being granted permission to be 'across the hall' so I assume they have some private, secure place for him to wait.