The idea that birds are descended from dinosaurs is nearly as old as evolution itself, first being proposed by Thomas Huxley in 1868 (Origin of the Species dates from 1859).
The only reason there was a competing evolutionary theory is because it was erroneously thought that birds have a clavicle and dinosaurs don't, so instead it was proposed that birds and dinosaurs have a common ancestor, and that dinosaurs lost the clavicle. Now that they have excavated many more bones paleontologists have since discovered therapod clavicles.
But now we know birds didn't descend from dinosaurs. They are dinosaurs. The ones that lived.
We don't "know" that, because that's not a category of thing we can know or not know. It's a matter of semantics of whether we consider birds dinosaurs, just like it's a matter of semantics of whether we consider people a kind of fish.
"Know" in science is used informally to mean "the preponderance of evidence supports this conclusion. Which could turn out to be wrong if enough contrary evidence is later found."
The scientific consensus today is that the evidence supports the idea that not all the dinosaurs died out when the Chicxulub comet struck 66 million years ago. The ones that could fly or quickly learn how to fly survived and even thrived, and their grandkids are in your back yard right now.
https://www.birdlife.org/news/2021/12/21/its-official-birds-...