A lot of animals are born "hardcoded" with most of the instincts they need to survive, so some behaviors are clearly innate.

And "how the brain learns from the incoming data streams" is, in part, driven a set of behaviors too.

A baby's eyes are trying to detect and track certain preset features long before the primary visual cortex learns to make sense of them. That's a behavior, and it exists for a reason. As the baby develops, the baby would try to seek out certain experiences to learn from them, which is a behavior that exists for a reason too.

There's a hypothesis that certain mental disorders are caused by this innate learning process going off course, but it's just a hypothesis, of the kind that's hard to prove conclusively.

Indeed. But most instincts involve elements of learning. Meaning the instincts may be stored using a much smaller number of bits than if they were stored as traditional IF-THEN-ELSE computer program.

For instance, the pattern the brain seeks to optimize to learn to work may be much smaller than the full algorithm for walking.

And if the brain learns quickly enough (and if a newborn animal started learning elements such as balance, moving legs, etc, before even being born), learning to walk may be learned in minutes instead of months.