Instinct shows up locally as emotion. An individual animal acts based on their emotional state, and their emotional state is governed by a set of rules deep in their brain of which they are not conscious, many of which are set by birth.

This is true of humans as well. We each make food selections based on what tastes good. We seek particular sexual partners because it feels good. We protect and raise kids because it makes us feel good to do so.

This causes all sorts of evolutionarily weird side effects like people treating pets like kids in order to access the same emotional state as parenting. Or beavers covering speakers with mud and sticks.

evolution uses whatever hook it can find to tune behavior. Brains of sufficient complexity have to learn, you can't fit even enough information in DNA to manually wire up a brain, and its hard enough to guess how a barin will end up being wired. you can attach a squirrels optic nerve to their auditory cortex and they'll learn to see. (I may have the animal wrong). You can grow a brain completely inside out that will function.

Instincts are deterministic, but learned behaviors.