First ask whether the commits are shared
Merge and rebase integrate work but preserve different historical facts. Before choosing, ask who depends on the commits you would change. A private topic branch gives you more freedom than a branch teammates have pulled.
A commit records a snapshot and parent links. Its ID depends on its contents and history. Replaying the same patch on a different parent still creates a different commit ID.
What a merge keeps
When branches diverge, a merge commit has two parents. Both histories remain connected. This makes the point where a feature joined the main branch explicit.
If the current branch is behind the target, Git can fast-forward: move the branch name without creating a commit. --no-ff asks for a merge commit even when that move is possible.
git switch main
git status
git merge --no-ff feature/idea
What a rebase replaces
Rebase replays changes unique to a branch onto a new base. The result can be linear, but those copied commits have new IDs. Old commits may remain in a local reflog temporarily; that is not a permanent backup.
git switch feature/idea
git status
git rebase main
Read the state before running either sequence. Uncommitted changes and conflicts deserve deliberate attention. Understand each side of a conflict before selecting a resolution; blindly keeping one side can discard the other change.
A force push is a separate decision
Rebasing locally does not automatically rewrite a remote branch. Publishing a rewritten branch may require a non-fast-forward update. --force-with-lease checks an expected remote state and is safer than unconditional force, but can still replace shared history. Coordinate first.
Try the same story twice
The example starts with a feature change and an independent main-branch change. Compare merge and rebase. Count parents and inspect the feature commit's ID. Load either result into the simulator, reset softly, and inspect the index and working-tree snapshots before trying a hard reset.
The simulator stops at conflicts and handles linear rebase histories. It never accesses a real repository or invents a conflict resolution.