Virginia Code lists ten factors a judge must consider in any custody decision. Read literally, they're abstract. In practice, they come down to a handful of very human questions.
Who has been doing the day-to-day parenting? Not who loves the child more — who takes them to the pediatrician, knows the teacher's name, and packs the lunch.
How stable is each home? Judges care about consistency: housing, work schedules, and who else lives there.
And crucially: will each parent support the child's relationship with the other parent? Nothing damages a custody case faster than being seen as the parent trying to cut the other one out.