My own experiences with social media and phone addiction leads me to believe its not JUST the content, but also (equally, perhaps moreso) the fact that teenagers AND adults with social media and phone addiction are just spending so much of their time absorbed into a screen scrolling and getting quick dopamine hits for hours and hours.
When that is your default that you revert to to distract yourself at almost any point of discomfort, slight boredom, lapse in focus, anxiety or feelings of sadness, all it does is just dampen those feelings momentarily. You live in almost a fugue state where those things just fester as you avoid them instead of learning to cope with them or respond to them in an agentic or healthy manner.
Working to cut out social media, reddit, youtube etc on my phone, leave my phone by our front door while at home, never have my phone in bed, and spend time writing/journaling my thoughts and actually engaging with my feelings of anxiety or depression or stress instead of avoiding them has been by far the most significant improvement on my mental health compared to many other things I've worked out.
This is definitely anecdotal, and its definitely my experience, but after the time i've spent working on these things and with what I see in the teenagers, young people, and friends in my life I feel very strongly about it.