As a person with ADHD, I can't say nicotine by any means reduces anxiety or helps to focus; on the contrary, the only time I don't vape for prolonged periods of time are when I'm concentrated on smth challenging, and otherwise it's somewhat like tapping my leg or spinning a pen with the addition of feeling worse when there's not enough nicotine in my body.

On the other hand, ADHD individuals are more susceptible to substance abuse disorders, so going in that craphole is not worth it. (e.g. https://www.nature.com/articles/s41386-019-0581-7)