A Question About the Soul and the Brain from an Islamic View

I’m not Muslim but I want to understand Islam better. I have an autistic brother, and it got me thinking: if we have a soul, why does his brain work differently from ours, and why does that make him who he is? I don’t know his soul, and no one knows mine. I’m curious how Muslims see this. If someone is born with a neurological condition, it shapes how they exist. If a person has an accident, they can lose their memory or their whole personality. A tiny change in the brain can change you, or even end who you are. It can change your consciousness completely. Since we can trace our thoughts to brain activity, it seems like there’s no 'floating man' as Avicenna described, just the brain. So if the brain is basically 'us,' why do Muslims say we have a soul? What are your thoughts? I’d love to hear.