My hottest “I have no data to back this up” take is that phones aren’t making young people less happy. What’s making them less happy is living in a world where everyone is having their reality defined by recommendation algorithms that can only care about whether something keeps you watching.
The generation behind this one talks a lot about the insecurity of the job market, what is "AI-proof", concerns with the changing climate, and in the US, lack of healthcare and concerns for affording things. My teen and his friends routinely talk about those things and how it impacts their future. They have a lot of concern.