The kids also used their phones constructively, for assignments, tracking school progress, Youtube how-to videos, contact with cousins, etc. They had social media accounts but they never became a focus.
At school they kept their phones on silent and in their bags.
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Fortunately three insights helped us:
1. Casual addiction is the intersection of two problems: an unhealthy addictive thing, and a lack of engaging or planned healthier alternatives.
Solve either one! But the latter problem is far tractable than the former.
2. There are only so many hours in a day, so solutions to the latter problem usually have a low complexity bound.
3. Social media isn't that rewarding for kids, once they have voluntarily invested their interest in longer term activities and challenges. Doom scrolling notably lacks any verifiable feeling of progress.
So solutions are pretty robust to ups and downs.
We kept our kids lives full of physical and real world social activities, family outings, social events at home, regularly eating times for talking and bonding, regular homework times, etc.