My simple understanding is that judges interpret and apply the law. But what about decision making? Am I right to say that decision making is part of their scope of duty of applying the law?
Different sorts of decisions at different levels. A trial judge makes decisions about what evidence is admissible, which witness may be called, and what questions may be asked, among other things. A trial judge may decide the entire case if there is no jury, or if there are grounds for overriding the jury, which a judge must also decide. Judges decide whether to issue search and arrest warrants or not. A judge decides on the sentence to impose on a convicted criminal. A judge decided whether to allow bail to an accused person, and if so how high it shall be. An appellate judge decides what standard of review should be applied to a case, and what laws and precedents are relevant. S/he then decides whether to uphold, reverse, or modify the decision of the court below.
Most of what a judge does, in fact, is make decisions.