I wouldn't put phones as the responsible one so generally. I view phones as tool to 'solve' specific problems, such as communication. Apps and websites such as Outlook (email apps with notifications in general), LinkedIn, Instagram, TikTok, maybe also Hackernews etc. pose more of the role of ruining our downtime, as they are designed that way.
At the end it would probably be about the lack of media competency, not knowing which apps and sites want to keep you as long as possible on their sites and which sources could benefit you better mentally.
And for children, aren't we adults (& probably also young adults) responsible to teach them about what media is and how to detect the bad apples?