The doctor at issue (Ronny Jackson) was the White House doctor while Trump was President. His office was in the White House. He travelled on Air Force one with Trump.

He was the doctor that gave an hour long press conference at the White House on Trump’s “excellent” health, saying Trump has “incredible genes.”

Trump then nominated him to head the VA, although he ended up withdrawing.  At that point Trump created a brand new position in the White House for him - Chief Medical Advisor to the President.

After he left the White House, Jackson became a US representative and has been one of Trump’s main supporters. Trump endorsed him when he ran. Jackson was just at Trump’s hush money trial with the other Texas republicans to provide him support.

This is not some random doctor Trump randomly saw once.

https://www.cbsnews.com/news/veterans-affairs-pick-ronny-jackson-impressed-trump-after-he-gave-glowing-health-report/

https://abc7amarillo.com/news/local/congressman-ronny-jackson-among-texas-republicans-at-trumps-side-as-defense-rests-in-hush-money-trial

https://en.m.wikipedia.org/wiki/Ronny_Jackson