Sprints and Branches Are Separate Concepts
[C]an we say that sprint and branch are almost the same things?
No, a Scrum Sprint and a Git branch are not at all the same things. While you can use an online Source Code Management (SCM) system like BitBucket or GitHub in the development process, there is no one-to-one mapping between the project management framework (in this case, Scrum) and your SCM workflow.
A Sprint is a time-boxed iteration with a cohesive Sprint Goal, and a set of features that comprise the team achieve the Sprint Goal. These features also comprise one or more increments of potentially-shippable value. These increments are the deliverables for each Sprint.
SCM workflows are negotiable processes that can vary between projects and teams. There is no canonical answer to the question of how one should use source code management across teams, projects, or organizations. That is really a question for the Development Team to address within the project framework, and is ultimately more of an engineering or architectural concern than a project management practice.