The Scrum team can estimate either during product backlog refinement or during sprint planning events.
The objective of the Estimation would be to consider the User Stories for the Sprint by Priority and by the ability of the team to deliver during the Time Box of the Sprint.
Agile teams usually estimate effort using a metric called story points. Story points are the abstract value that takes into account the relative complexity and size of a task. Teams tend not to estimate based on person hours, but instead use Story points – a concept inherited from Extreme Programming (XP).

The definition of Done is about building the product “right,” and Acceptance Criteria is about developing the “right” product.

According to the 2020 Scrum Guide, “The entire Scrum Team is accountable for creating a valuable, useful Increment every Sprint”. The definition of Done (DoD) is designed as a checklist, and it's used to check every user story for completeness or product backlog item. In other words, DoD is made to apply to all items in the product backlog, not just a single story.

The Acceptance Criteria of a User Story consist of a set of Test Scenarios that are to be met to confirm that the software is working as expected. It:

• applies to one Story/PBI,
• is different for each story,
• is used as a way of communication for all those involved to ensure that all story requirements have been met.

Therefore the product owner is responsible for specifying acceptance criteria and user stories.

A new Sprint starts immediately after the conclusion of the previous sprint, and each Sprint typically begins with Sprint Planning and ends with Sprint Review & Sprint Retrospective.

Agile scrum artifacts are information that a scrum team and stakeholders use to detail the product being developed, actions to produce it, and the actions performed during the project. The main agile scrum artifacts are product backlog, sprint backlog, and increments.

The term artifact is often associated with archaeological ruins and ancient relics. Yet in software development, the term artifact refers to key information needed during the development of a product.

Agile scrum artifacts are information that a scrum team and stakeholders use to detail the product being developed, actions to produce it, and the actions performed during the project. These artifacts provide metadata points that give insight into the performance of a sprint. They are essential tools for every scrum team since they enable core scrum attributes of transparency, inspection, and adaption.

Scrum Master Responsibilities:
1. Implement Project Management/Best Practices;
2. Keep all Parties on Track and Informed;
3. Introduce Agile Engineering Practices;
4. Coach Team Members;
5. Host Daily Stand-up Meetings;
6. Assist the Product Owner With the Product Backlog. It is the product owner’s responsibility to create and maintain the product backlog. The Scrum Master helps the product owner refine and maintain the backlog by using information gathered from standup meetings. They schedule review meetings and prioritize work on user stories.
7. Remove Roadblocks;
8. Teach Scrum Practices and Principles.

A stakeholder is "a person external to the Scrum Team with a specific interest in and knowledge of a product that is required for incremental discovery. Represented by the Product Owner and actively engaged with the Scrum Team at Sprint Review.