Parenting Struggles

I am struggling with setting (and enforcing) clear screen rules for older kids. 

They have their school computers and I have no idea how in the world I am supposed to keep track when they are doing actual work versus when they are consuming ghastly quantities of YouTube Shorts. The 8th grader has a phone - and again, it's a struggle to know when he is using it to communicate (rarely) versus watch random stuff. Also, he is often in his room, with door closed, doing his own thing - how do I balance giving him privacy with making sure  he does not spend all his free time attached to screens? (Same with the 7th grader except she doesn't have a phone yet).

They are good kids and I trust them (mostly) but I feel, for the first time, like what I am doing is not enough. But I don't know what else to do.

When kids were younger (and this is still the case with our 9 year old) - it was much easier because they didn't have any devices. The played with toys, they read, and they had limited parent-supervised screen time on weekends (a movie, or husband's ancient Nintendo, or Minecraft on my computer).

How do I figure it out? How do I learn what's best for my kids - besides trial-and-error? 

There is such a range of parenting approaches - how to know what's right for our family?

I don't want to be super-controlling... but I don't want to be oblivious... and it seems like it's all too easy to be oblivious. With older kids, what they tell me about school and friends is only a small fraction of what actually happens and does not necessarily paint a true picture of their reality. 

I want to be aware of any problems (loneliness, toxic friends, drugs, bullying) - but it's not like they'll just sit down and tell me. The fact that they are not telling me about any problems in their lives may mean that they are happy and everything is fine... or it may mean they are just not telling me stuff.