I think ADHD (I have it too) is best described as a quality of life impacting lack of executive function control in a given context. It is a sort of state of longterm executive function drought in a mental landscape of plants that really need more executive function to grow then they are getting so it becomes a constant process of triage.
The thing is, that is a description of an end state, not a mechanism really. If you sleep deprive "neurotypical" people you can induce a temporary pseudo-state of ADHD where they begin to lose executive function ability. There are many ways for someone to be pushed into executive function burnout too.
In my mind ADHD has to be spoken of from that framing because it has always been a condition defined by symptoms. ADHD is the set of people who cannot be reasonably expected to escape a state of executive function drought symptoms given the context they are forced to live amid. It almost would defy common sense for there only to be one root cause of people ending up there over and over again, it is a place to me, a physical experience any body could potentially encounter at least briefly. The point is not that all ADHD people have some magic physical identifier that unites them all, the point to the concept of ADHD is about caring for people who cannot fulfill the volume of executive function demands placed on them in daily life no matter what continually places them in that state.