My understanding, but perhaps the OP will correct me, is that ADHD means the sufferer has lower-than-average levels of dopamine or difficulties with dopamine processing. So "less dopamine" is not the answer and screens have nothing to do with ADHD except that people seek them out as a means of getting the "hit" they need.
NT people have something that those with ADHD do not. And that is sufficient levels of certain chemicals that have a stimulant effect and act as neurotransmitters in the brain. That means they get a "reward response" from everyday activities that is lacking in those with ADHD.
Left undiagnosed, therefore, ADHD sufferers will seek out alternative methods of getting what their brains need - whether sensory stimulation, screen time, food, exercise or, in extreme cases, alcohol or drugs.
Medication involves artificially replicating the chemicals which NT people already have in their brains and which allows them to better regulate their behaviour and responses.
I suspect my older DC also has ADHD and I let him have screen-time in small doses in between doing homework so he gets the "hit" he needs and can then concentrate on the homework. I also encourage frequent screen breaks and going outside on the climbing-frame with bare feet in between doing questions as that works to improve his concentration. He doesn't need that at school so much because the environment there seems to put him on hyper-alert anyway. But we have real problems with concentration at home.