There are degrees of monitoring. This is basically my approach:
no smartphone before 12yrs old
no computer, tablet, smartphone or similar in their bedroom before 16yrs old, specifically at night.
family link (android) installed, with tighter controls the younger they are. This is discussed and also explained as a measure to protect them
if they are under 16, we create social media accounts together and discuss what is appropriate, what can/should be public/private etc.
That is basically it. A lot of it is being around, available and approachable. It’s not perfect, but it has several layers of protection, and is built around creating trust and teaching valuable media skills.
If I had to pick one, I’d say the ‘no internet devices in bedrooms’ would be the most valuable one. Because of that, I know what games my kids play, they can deconnect at night, and it’s fairly easy to enforce.