Both git merge and git rebase exist to do the same job — integrate changes from one branch into another. They produce different histories because they go about it in fundamentally different ways, and understanding that difference is what stops you from corrupting a shared branch.
What merge actually does
A merge takes the work from two branches and joins them with a single new commit — the merge commit. That commit is unusual because it has two parents: the tip of the branch you were on, and the tip of the branch you pulled in. Every commit on both branches stays exactly where it was, untouched. Git just adds one node on top that says "these two lines of history came together here."
Suppose you branched feature off main, and while you worked, someone else pushed commits to main. When you run git merge main from your feature branch, Git finds the common ancestor of the two branches, computes the changes on each side since then, combines them, and records the result as the merge commit.
main: A---B---C---D
