My take!
Imagine the fully-working brain has a mass of tiny lights, all of them on, with pathways in between. Dementia causes some of those pathways to get blocked but there are still enough 'alternative routes' to connect the lights. Other times, the re-routing gets too long or too complex and the connection doesn't get made. Then some of the lights go out. And like a machine, some go out immediately and permanently. An example might be when the person's body clock no longer works and they cannot distinguish day from night or lose their sense of time. When this happens, it rarely comes back - must be a big light! But other lights flicker on and off for a while before going out for good.
OK, this isn't exactly how brains work - it's just an illustration, but it made some sense of my mum's decline.
I was always fascinated by the effects of dementia on the brain. (And it's slightly different for everyone of course.) I learned a lot about memory and cognition, none of which helped my mum unfortunately