Every ORGANIZATION is different, they have their own HIERARCHY. Scrum does not provide any ground rule on the reporting structure for the product owner. In large organizations where the product is fairly big, 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. In this case, the product owners will be reporting to the product managers. But as stated before, there is no set criteria or hierarchy being followed at the organizations. Some even align the associates as per their position e.g. product owner at the lead level will REPORT to someone at the manager level.

“As a Product Owner, your JOB is not to manage 'resources' or 'tasks'. Your job is to maximize the value of your product! To create those features that deliver the most value for the products' users! In order to maximize the value of your product, you don't have to manage stuff like tasks, what people do on a daily basis, what the progress of the team is in a Sprint.”- Scrum.org