There are a lot of opinons out there, so let me just throw mine into the ring as well.
What's not a Product Owner?The Scrum Guide is quite clear that a Product Owner and a Business Analyst are not the same roles. Neither is a requirements engineer or a project manager a Product Owner. A Product Owner is not responsible for "writing user stories". Neither proposing solutions, architecture or design. Nor for tracking developers' progress, the quality of their work or the approach they use in generating results.
Likewise, there's a huge misunderstanding that the PO is a kind of go-between, mediator, between stakeholders and developers. Oddly enough, the Scrum Guide doesn't say any of that.
Indeed, a PO who acts in any way as above is more likely to negatively impact the outcome than to add value.
Then what is a Product Owner?When taking a close look at the Guide, the Product Owner can delegate almost everything to the Development team. They only need to make sure that transparency on a product level exists, that developers know what is important and why. How they do this - no constraints.
In one sentence, the PO can be called "value maximizer". How do they do that? They learn and understand customer needs, problem statements and expected benefits from having such a need met by solving said problem. To be most effective, it helps if they have an understanding of what their development team is capable of, have a knack on linking the solutions of the team back to the problems that exist and can make clear business-oriented decisions based on cost and ROI.
Like customers own the "problem space", developers own the "solution space". The Product Owner brings these two spaces together by linking them via value and priority. No more, no less. The more the development team can contribute here, the more the Product Owner can focus on exploring further needs that can be met to make the product more valuable.
So here's a short infographic as a summary: