There are lots of inconsistencies, with some studies finding some differences that then don't turn up in other research. However, especially if we look at activity and connectivity, the frontal cortex is a big focus of interest, as is cingulate cortex, and corpus callosum, and even parts of the striatum. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9853532/#:~:text=This%20study%20showed%20that%20abnormal,with%20ADHD%20pending%20future%20research.

To oversimplify the cortical thickness issue, from childhood on, our cortices are being thinned out, and those with ADHD show slower rates, or delayed maturation, of this thinning process, particularly in frontal cortex.