Every organization has its own reporting hierarchy. Scrum does not provide any mandatory ground rule on the reporting structure for the product owner. In a large organization where the product is fairly big and they use agile frameworks like SAFe, they have product managers at the highest level, who are the main owners of the product. At the team level, they have product owners who constantly stay in touch with the product managers and report to them. But again, there is no set criteria or hierarchy being followed at the organizations.