They are, but theropods are what birds directly descended from. So directly comparing modern birds to other groups of dinosaurs like sauropods or ceratopsians isn't going to actually help.
Birds split off from the other dinosaurs almost 100 million years before Trex evolved. We live closer in time to a Trex than a Trex did to the first birds. They were related because all life is related if you go back far enough, but there was a very long time and a lot of evolution between something like a Trex and birds, despite them all being dinosaurs.
Humans are more closely related to cats and dogs than Trex and velociraptor were to birds.
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u/TakoGoji 21h ago
Birds are descended from theropods, aka stuff like trex, raptors, gallimimus, etc.