What's the source on significant over diagnoses?

I don't believe I said the words "significant over diagnoses". However, for example for ADHD there was a pretty good article in the New York Times last year (scroll down, the page has a bunch of whitespace at the top): https://web.archive.org/web/20250414202754/https://www.nytimes.com/2025/04/13/magazine/adhd-medication-treatment-research.html

Also, how can sacountry workout universal healthcare have such excessive diagnoses when so many aren't even getting the healthcare at all?

These conditions are often diagnosed during childhood and youth, where most Americans are AFAIK covered by national programs as well as their parents' insurance.

For adult diagnoses, there's a selection bias towards people who have self-diagnosed and seek confirmation, which will logically lead to a diagnosis rate among patients that exceeds the true incidence of the condition in the general population (due to the selection bias), as well as a slightly or somewhat increased apparent incidence of the condition in the general population (due to people who self-diagnosed without actually qualifying for a diagnosis, but read enough about the condition to effectively lie their way to a diagnosis).