Sprint planning defines a sprint backlog.
The product owner proposes an opportunity for increasing product/stakeholder value and the team sets a goal to meet that. Full team discuss options and historic capacity to determine what can be done to meet the goal. A plan that meets the Definition of Done is set for one or more increments. Developers break down backlog items into achievable pieces and decide how to define the increment with those pieces. Other stakeholders may join to help set requirements and goals.rS2
Ken Schwaber and Jeff Sutherland, The Scrum Guide: The Definitive Guide to Scrum: The Rules of the Game, 2020, https://scrumguides.org/docs/scrumguide/v2020/2020-Scrum-Guide-US.pdf. (See notes.)