It's OK for a Product Owner or a Scrum Master to be also a Developer even though it's not recommended.

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.

The Sprint Backlog includes items from the Product Backlog (the prioritized list of requirements for the project) that are deemed most important and feasible to complete within the Sprint time frame. The Sprint Backlog serves as a plan of action for the Developers and provides a clear understanding of what needs to be accomplished during the sprint.

The Sprint Backlog is composed of the Sprint Goal (why), the set of Product Backlog items selected for the Sprint (what), as well as an actionable plan for delivering the Increment (how).

The Sprint Backlog is a plan by and for the Developers. It is a highly visible, real-time picture of the work that the Developers plan to accomplish during the Sprint in order to achieve the Sprint Goal. Consequently, the Sprint Backlog is updated throughout the Sprint as more is learned. It should have enough detail that they can inspect their progress in the Daily Scrum.

Nothing happens between two Sprints; the next one starts immediately after the conclusion of the previous.

The fundamental unit of Scrum is a small team of people, a Scrum Team. The Scrum Team consists of one Scrum Master, one Product Owner, and Developers.

"Scrum combines four formal events for inspection and adaptation within a containing event, the Sprint. These events work because they implement the empirical Scrum pillars of transparency, inspection, and adaptation."

- The Scrum Guide (TM) of 2020.

In Scrum, it is the Developers who are responsible for estimating the effort required to complete the Product Backlog Items during Sprint Planning. However, it is important to note that the Product Owner, as the person responsible for maximizing the value of the product, should provide the Developers with clear and detailed Product Backlog Items to enable them to estimate the effort required accurately.

The Developers should work together to estimate the effort required for each Product Backlog Item, using techniques such as relative sizing, story points, or other methods agreed upon by the team. The estimates should be transparent to the entire Scrum Team to facilitate effective Sprint Planning and progress tracking.

It is important to note that while the Developers are responsible for estimating the effort required for Product Backlog Items, the Product Owner should still be actively involved in the estimation process. The Product Owner can provide additional context and clarification to the Developers, which can help them to estimate the effort required more accurately. Additionally, the Product Owner can use the estimates provided by the Developers to prioritize the Product Backlog Items based on their value and the effort required to complete them.

The Product Backlog lists all features, functions, requirements, enhancements, and fixes that constitute the changes to be made to the product in future releases. Product Backlog items have the attributes of a description, order and estimate.

A spike indicates added work. The Product Owner cannot add new work without the Developers consent.

The Scrum Team is accountable for releasing the most valuable product possible. The Developers are accountable for doing the work. The Scrum Master is accountable for ensuring Scrum is adopted and used properly.