Chicken is equally close to T. Rex as all other birds. They all go back to the same common bird-ancestor who was a relative of T. Rex.

T. rex is more closely related to birds than to all non-theropod dinosaurs.

T. rex also lived closer to us in time than to Stegosaurus.

Same reason why humans don’t actually descend from monkeys, both monkeys and humans descend instead from a common ancestor that no longer exists

But that ancestor was a monkey! We are monkey, you can't evolve your way out of a clade.

This is why, IMO, Digimon could be a better representation of evolution than Pokémon, since one Digimon can evolve into many different forms, so that kitten could become a fridge, but also a dragon or a knight with cannons

But that is specifically not how evolution works. You can't evolve your way out of a clade, everything a cat would evolve into, would also be a cat. Maybe a weird cat, maybe a whale-cat (like whales are still ungulates, even if they are really weird ones) but still a cat. You can't evolve out of your ancestry.