A custody case involves several people with different kinds of authority, and confusion about who decides what is common. Three groups typically hold authority in a case (the parents, their attorneys, and the judge), and each one's authority is narrower than it first appears.
Parents decide most of it, usually without a trial
The large majority of custody cases settle rather than go to trial. Parents, often through their attorneys, negotiate a parenting plan covering the schedule, decision-making, and other terms, and the court then adopts that agreement as an order. In that sense, parents hold more authority over the outcome than the phrase "the judge decides custody" suggests. What attorneys typically advise on is what terms are realistic, what a court would likely do if the case did not settle, and what language will hold up if a dispute arises later.
Attorneys advise and negotiate, within limits set by the client
A family law attorney's authority is also bounded. Under the ethical rules that govern lawyers in every state, an attorney is expected to abide by a client's decisions about the objectives of representation, including whether to accept a settlement, while the attorney generally controls the tactical and technical means of pursuing those objectives, such as which motions to file or how to conduct negotiations. An attorney cannot settle a case, or refuse a settlement, against a client's express wishes.
A judge decides only what the parents cannot resolve, on a limited record
When parents cannot agree, a judge decides the disputed issues. That authority is formally the broadest in the room, but it operates under a real constraint: a judge knows only what the case file, testimony, and evidence put in front of them, often after a small number of hearings. Reports on family court caseloads describe judges managing large dockets with limited time per case, which is part of why documentation and clear presentation of facts weigh so heavily on outcomes. Research funded by the National Institute of Justice, studying a decade of custody filings involving intimate partner violence, found that legal representation measurably changed which facts reached the judge and how those facts were weighed. Represented parents were substantially more likely to secure protective custody terms than unrepresented parents raising the same underlying concerns. Separately, research from the Institute for the Advancement of the American Legal System found that self-represented parties often were not unsure of their own facts, but unsure which of those facts the court needed to hear and how to present them.
Where this leaves a parent trying to figure out "who decides"
The practical answer is layered: a parent retains real authority over whether to settle and on what terms, an attorney's authority is to advise and negotiate within the objectives the client sets, and a judge's authority, while formally the final word, is exercised on whatever record the case has produced by the time a decision is required. Questions about how a particular judge tends to handle a particular issue, or how much weight a specific piece of evidence is likely to carry, are exactly the kind of question a licensed family law attorney in the relevant jurisdiction is positioned to answer.