It can be both though, there's ADHD as it is currently understood as a genetic/neurological issue where the dopaminergic system is impacted, and creates all the effects experienced by ADHD sufferers.

There might also be environmental issues, like you mention, that could be creating similar issues but not from the point of a genetic neurological divergence.

That is an important distinction since the treatment for one cause is very different to another.

The incidence rates increasing are very correlated to better diagnoses, 30 years ago most people with ADHD wouldn't be diagnosed unless they fit into the extreme cases of it. That was my experience, as a kid I was very bright but considered "lazy" since I couldn't focus as my peers did out of school, was always interested during classes, and excelled in school even though I could never do my homework or other assignments, constantly forgot important deadlines, it was always stressful but I found ways to manage it. Thirty years later and I fit in the criteria, no idea how my life would've turned out if I knew about it before, my coping/management mechanisms ended up being to lean into the stress and harness it, it made me achieve things but also created a whole other host of issues with anxiety later in life.