Scrum Guide
The Developers who will be doing the work are responsible for the sizing. The Product Owner may influence the Developers by helping them understand and select trade-offs.

Having a product vision is important because it provides a common understanding of the direction the organization wants to take the product. For example, it helps you develop a better product roadmap, it improves your decision-making throughout the development process, it helps you align teams and stakeholders, etc.
Two characteristics of a well express vision:
1. It is actionable and directive. It should be easy to make decisions based on the product vision.
2. It is clear and inspiring. It helps people involved in the product understand why they are building the product and the impact their work has on the product.
A characteristic of a poorly expressed product vision is one that isn't measurable. Because the visualization is lacking it leads to a product vision that doesn't feel real
For this answer, observe the focus is on product vision, which is not in the Scrum guide, however, it is a valid area for a product, and it is explained the importance of a product vision, and also provided a few examples for a well and poor characteristic.

Every role and topic listed could be helpful for the Product Owner to gain insights and obtain feedback

Scrum Guide
The Product Owner may represent the needs of many stakeholders in the Product Backlog. Those wanting to change the Product Backlog can do so by trying to convince the Product Owner.
Also
A product is a vehicle to deliver value. It has a clear boundary, known stakeholders, well-defined users or customers. A product could be a service, a physical product, or something more abstract.

Scrum Guide
The purpose of the Sprint Review is to inspect the outcome of the Sprint and determine future adaptations. The Scrum Team presents the results of their work to key stakeholders and progress toward the Product Goal is discussed.