Mainly, scientists increasingly believe brain fog happens when cells that are involved in response to an infection make their way to atypical places, such as the brain.

A key finding emerged when researchers autopsied brains of COVID victims, discovering certain cells that shouldn't have been there. These large cells, known as megakaryocytes, might be taking up precious space, leaving less room for blood to pass to the brain.

This phenomenon might be unique to COVID, according to David Nauen, MD, PH.D, a professor of pathology at the Johns Hopkins University School of Medicine.
"To have megakaryocytes in the brain has never been seen before, I couldn't find any reference in my search this past summer with megakaryocytes noted in human brain capillaries.

"The brain cortex is richly vascularized with capillaries, if some proportion of capillaries are blocked or occluded by these cells, it blocks some level of blood flow.

"the findings on autopsy suggest that the burden could be significant,"

https://abcnews.go.com/Health/scientists-inch-closer-explaining-mysterious-brain-fog-symptom/story?id=76184080