First, the mind is not the brain and second, people tend to see the mind–brain relationship in terms of the cultural conceptions that matter to them:
The means by which people with major parts of their brains removed maintain function are not understood. It’s nonsense to say, as some do, that “The brain is massively parallel and recursive and functions under network rules and laws.” That’s typical neuroscience gibberish. The fact is that neuroscientists study the brain using network theory, and… surprise!… the brain seems to be a network.
The ancient Greeks studied the brain according to caloric theory, and… surprise!… the brain seemed to be a heat generator.
The 19th century physiologists studied the brain using mechanical concepts and… surprise!… the brain seemed like a machine…
My computer is a network and has redundancy. But if I cut it in half with a chainsaw it sure as hell won’t work. Neuroscientists are just making stuff up. It’s confabulation, not science.
News, “A neurosurgeon on why some people function with only half a brain” at Mind Matters News (August 19, 2022)
Michael Egnor also notes, “The diseased hemisphere was never particularly functional anyway, and brain function in the healthy hemisphere was probably adequate for many years prior to the surgery.”
But that only answers some of the questions.
You may also wish to read: People with half their brains removed do well on psych tests. In a recent study, adults who had had hemispherectomies as children — to combat severe epilepsy — performed within 10% of other study subjects on face and word recognition. Findings like this are a challenge to those who insist that the mind is simply what the brain does. The mind may not be split or removed when the brain is.