To wonder how it is decided who is the main carer of a child?

[TheDetective]

If a childs parents separate, residence normally goes to the main carer - right?

But how is that decided in this day and age of equal parenting? Is is the parent who stays at home? The parent who works less hours? The parent who gets up with the child in the night?

How? Who can decide this?

What if the parents work the same number of hours, and share parenting completely? Do they get shared residence?

I just don't see how it works. AIBU?

I know families where they work a rolling fortnight, because 14 can neatly be divided into 2! They work a 2,3,3,2 pattern. The children move between two houses every 2 or 3 days. It's sounds complicated but seems to be quite common. It only works well if both parents live relatively near each other/schools/friends etc.
Both parents genuinely have the children 50% of the time.
Can make it difficult deciding who gets child benefit and if both parents work there may not be any maintenance.