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So as a heap, all node's are less than its children, but once I print the array: 3,6,7,15,16,8, It should be: 3,6,7,8,15,16. Why is this point never brought up whenever I heart efficiency of a heap to build a PQ (max or min PQ)?

A heap satisfies the heap property - which defines the relationship between parent and child, there is nothing within this property that suggests sibling order. Hence represented within an array a heap may not be a sorted array. A heap as a priority queue is efficient given its log(n) complexity for inserts and deletes.