> You can't say "ADHD is this" and point at a definitive cause

There's plenty of existing and on-going research that demonstrates differences in brain structure and development in people clinically diagnosed with ADHD. In other words, ADHD is caused by parts of your brain being underdeveloped or damaged. There is on-going research applying machine learning to MRI brain scans to try to predict ADHD through brain scans rather than only clinical diagnosis.