I guess its tempting to take like MRI/ECG style studies and draw fundamental conclusions from them about the distinction between "personality" and "disorder" but I think its good to bear in mind at least three things:
1. a lot of these studies suck. Brain imaging is very hard, the interpretation and analysis of the results involves lots of degrees of freedom, the study sizes are typically not as big as you'd like, and most of the results are only really visible in aggregate. I do not give much credence to them, as a scientist. One way to think of this is that if someone separates two groups of people into "ADHD" and "NOT ADHD" and you average their MRIs you might detect a difference in the two groups. But one person's MRI would be almost useless to assign them to one of the two groups. You could certainly try it, but it would not be very effective.
2. Literally every difference in behavior between two people or between a person and themselves at a different time is necessarily reflected in a difference in brain behavior, at least if you buy the materialist paradigm that brain -> mind or at least brain == mind. Thus, you would expect differences in personality to show up in MRI scans as well as differences which rise to the level of "disorder."
3. The brain isn't made up of "specific parts with specific functions." While its certainly true that we can roughly map different areas to different functions, its really not separable in any way that (for example) a human designed machine might be. We cannot remove and replace your "attention center" and it doesn't really mean anything to talk about it without all the rest of your brain. The part/whole relation is bullshit in all contexts (in my opinion as a mereological nihilist) but especially in neuroscience.
I guess its sort of a useful rhetorical frame to point to physical differences between brains as some kind of determination of the distinction between "mere" personality differences and "disorders" but I just don't think it makes sense in a fundamental way.
I'm a person with ADHD and Autism diagnoses and I think they are handy things to use from time to time, I think of them as entirely relational descriptions pertaining to my position with respect to the world, not fundamental ontological categories. On the other hand, I think of essentially everything as relational and I don't really believe in fundamental ontological categories so maybe I'm the fucked up one.