the first one that comes to mind is that birds can fly?

https://en.wikipedia.org/wiki/Archaeopteryx

Another is that while all birds descended from dinosaurs, not all dinosaurs descended from birds - in fact none did afaik.

Which is why I said all birds are dinosaurs, not all dinosaurs are birds. What does that have to do with anything?

Another example like this is that all large mammals descended originally from single-celled organisms, but not all single-celled organisms descended from multicellular ones, in fact most (probably literally all?) did not.

Likewise, just as all single-celled + multicellular eukaryotes belong to a single monophyletic clade, but there are ENORMOUS differences between them (fungi vs. plants vs. humans), so too do dinosaurs and avians belong to the same monophyletic clade, for all that that means.

Nothing to do with theropods vs. birds.

Which MEANS then that the word “dinosaur” itself needed to be redefined, after that discovery about birds being part of the same group. So they did that:

Your link says that is a "more handy general definition," not a scientific one. Furthermore, the very next paragraph of your link says-

Our definition above does leave out something very important: It is now known that birds evolved from small carnivorous dinosaurs during the Jurassic. Therefore, dinosaurs are not extinct, they are not confined to the land, and we would not think of many true dinosaurs as “reptiles”. Because modern birds are so distinct from reptiles, and became very specialized for flight early on, many paleontologists find it useful to distinguish birds from the other dinosaurs. If you go through the scientific literature, you might see something like “non-avian dinosaur”. This just means the scientist is excluding birds.

Did you even read it? It literally contradicts your claim. It can't contradict your claim any more clearly. And yet you use it to make your point that birds are not dinosaurs?

And the paragraph after that also talks about birds, citing why paleontologists use the term “non-avian theropods” to carefully distinguish the true reptiles from the birds that came out from their midst.

Ah, so birds are theropods.

So theropods both are and are not dinosaurs?

Yet again- paleontologists disagree with you. Do you have a degree in their field?