At what level should the product owner detail the work?

While the Product Owner is accountable for effective backlog management, which does include creating and clearly communicating Product Backlog items, responsibility can be delegated and shared within the Scrum Team.

Refinement, while not a Scrum Event, is an important practice where the team collaborates, ask questions and provides a level of detail that supports PBIs being ready enough to be pulled into a Sprint.

Is backlog refinement occurring and is it a team activity that includes Developers?Regarding…

Developers say that there is not much detail in the PBIs written by the product owner.

What, as a self-managing team, will they do to help with this? How can they contribute to the refinement of PBIs so they contain enough information for them to work effectively?

In addition to the great questions asked by Ryan about focusing on refinement, I'll add this one since user stories were mentioned. What is the communication and collaboration like during the Sprint?

The user story is a promise whereby Developers have a conversation with the Product Owner and ask questions throughout the Sprint, and the Product Owner promises to be available for the Developers to have those conversations.

I find that talking about the PBIs rather than relying on abstract ideas written down is always better.

Thank you for your responses.

What, as a self-managing team, will they do to help with this? How can they contribute to the refinement of PBIs so they contain enough information for them to work effectively?

Dear Ryan, you also know that the product owner should have 100% control over the product, but unfortunately this may not be the case in the real world. There may be problems in the product owner's ability to fully understand the product-related needs and narrate them. This is actually the example we experience in our company.

My question is at this point. At a point where the product owner cannot determine the needs of a PBI, can't support be requested from developers members in detailing this work? This can be a spike or an analysis task, whatever we call it

What I actually want to ask here is this: If the product owner cannot fully write the acceptance criteria for a PBI or understand the requirements well, he/she should not include this work in the sprint

Think of it the other way around. If the Product Owner could specify the requirements to that level of detail, there'd be a problem. Why bother using Scrum at all? We'd already know, in depth, what needs to be built. There'd be no need to engage in experimentation Sprint by Sprint and to apply validated learning.

The important thing is that work should be ready for Sprint Planning. As long as the Developers are satisfied that this work can be Done in a Sprint, that's enough. From their perspective it's a matter of being able to meet the Definition of Done. That's it. The depth of analysis does not necessarily need to go any deeper than that. Experiment, and then find out.

Let's also not forget that the Product Owner's competencies are not without an impact on the work in Scrum. One of the problems organizations are facing when adopting Scrum is filling the Product owner position properly. 

There is a huge variation of possible products out there in various domains. Some products are easy to define with a human user base, and some are abstract, technical products. It is not always easy to find the boundary between the Product Owner, Developers, Stakeholders.

Let's say the product is the networking card driver. In order to be an effective PO one should have pretty good knowledge about the networking domain, the capabilities and features of the hardware for which the drivers are being built, and so on.

Now, what is the Scrum Master role when facing struggeling Product Owner?

It is a challenge, which requires emphaty, diplomacy, skillful coaching, challaning work with Scrum Team and the organization. 

"What I actually want to ask here is this: If the product owner cannot fully write the acceptance criteria for a PBI or understand the requirements well, he/she should not include this work in the sprint, but can he request support from the developers members in that sprint?"

I am sorry for very short and strict answer but YES!

He SHOULD request support: both during Product backlog refinement(which should be done together with developers) and at actual retrospective.

How they will facilitate this support - is up to Scrum team, especially to SM, who SHOULD do his best to assist PO in such situation.

There may be problems in the product owner's ability to fully understand the product-related needs and narrate them.

No. That is exactly their job. That is what being a PO is about.

It could be the case, and it sounds a little bit like it, that the "real" PO is someone else. But even so, how can one create backlog items without understanding what they are about? A PO needs to have a good understanding of the domain and the market.

I don't understand the last paragraph of your question.