Overview
Creating a branch does not move you onto it automatically when using git branch <name>. You must switch with git checkout <name>.
Changes must be committed to become part of a branch's history. Uncommitted work is still working-directory state and can block a checkout if the target branch would overwrite it.
A branch does not affect main until its commits are merged, rebased, or otherwise integrated.
Branch isolation reduces risk, but it also creates multiple lines that eventually need coordination. That coordination is the purpose of merging, conflict resolution, remote branches, and pull requests.