Think about yourself wouldn’t you want that thing always go your way? Of course you do. The same principle applies to children as well.
Even the youngest children are already using the “separate and conquer” strategy. They want a treat; mommy said no they go to daddy hoping for a different answer. Once they got what they want they are in control. This situation become even easier for them when the parents are separated.
It is very complicated since each one of us has a different priorities and different value system.
The solution is valid for married and separated couple: separated couple are still the parents of their children and they need to perform as a solid unit as parent.
One of the ways to form this unit is to have a long and open conversation between the parents in which you lay out the important values and rules in which you are going to back up each other. For example you can both decide on the amount of screen time the child is having and to enforce it whenever you are in charge.
Only when both parents are on the same page some of the values and rules can be internalize by the children.
Make the effort to communicate with the other parent it is worth it.