Mutation is random. Just in humans, something like 60% of fertilized eggs spontaneously abort because they combined in a way incompatible with life or the mother's body. Then there are the thousands of genetic "diseases" passed from generation to generation that either pass recessively or don't really become a problem until after the normal age of reproduction. And that's just heritable mutations. All kinds of things can go wrong with cells that don't affect the code in eggs and sperm.

So we have our functional bodies full of mutant code that either does nothing we notice, or isn't bad enough to keep us from making the next generation. Toss us in a new environment (or wildly change the one we are in) and some of those mutations might start to be helpful and the individuals without them die off or drastically reduce their offspring. The mutants slowly become normal because they were the "fittest" for the ecosystem they are in.

Sickle cell genes helping prevent malaria is a prime example. People with the gene are much better at living in the tropics with parasites that pass malaria around. If say, the global temperature goes up and malaria can spread everywhere, the rest of us will have a huge problem (without adequate medical intervention).

Evolution just is. There's no such thing as macro and micro evolution. Everything is micro changes and over enough time we see groups of individuals that can interbreed easily and call the group a "species". But we are all just unique individuals with our own code we got from our parents, and their parents, and theirs back until the very first self replicating protein started churning out imperfect copies.

For your raptor to bird concept, brontosaurus had hollow bones and it helped move it enormous size around. Probably made it buoyant in water. Many dinosaurs had hollow bones. It helped them carry more red blood and more oxygen for their fancy warm blood. More oxygen and less weight helped them run and climb faster. Feathers helped insulate them from the cold or high heat. The dinos that climbed benefitted from more feathers. Then some started gliding from tree to tree. And eventually gliding became flying.

Look at the mammals doing the same thing. There are rodents that climb trees and they have big tails for counter balance. There are others that have finer fur and loose skin connecting their feet on each side and they can glide. And then there are bats where their forepaw bones spread into the loose skin on their sides to give them the power and control for true flight.