Also they're not properly reffered to as braincells. They're neurons. The average human has 86 Billion Neurons. Over time, mental illnesses such as dementia can make you lose your ability to control your neurons. Think of a pipe. They control something flowing through it. now imagine if every few clusters 1 pipe can hold a single pixel or yes/no switch.
That's vague and likely inaccurate but this is essentially what neurons are