Most dinosaurs could not put their hands palms-down, because the lower arm bones were not capable of twisting long-ways over one another. So Tyrannosaurus rex and most other dinosaurs would have held their hands palms-inward, as if they were getting ready to clap, not palms-down.
The rotation of the forearm that allows us to turn our hands palm-up or palm-down is called pronation and supination: https://commons.wikimedia.org/wiki/File:Pronation_and_supination.jpg
However, T. rex might have been able to turn their hands palm-upward: https://www.livescience.com/63858-t-rex-dinosaur-arms.html
Most dinosaurs could not put their hands palms-down, because the lower arm bones were not capable of twisting long-ways over one another. So Tyrannosaurus rex and most other dinosaurs would have held their hands palms-inward, as if they were getting ready to clap, not palms-down.
Source: https://www.reddit.com/r/askscience/comments/13xe18/how_do_paleontologists_know_that_they_put_the/
The rotation of the forearm that allows us to turn our hands palm-up or palm-down is called pronation and supination: https://commons.wikimedia.org/wiki/File:Pronation_and_supination.jpg
However, T. rex might have been able to turn their hands palm-upward: https://www.livescience.com/63858-t-rex-dinosaur-arms.html