But what happens if you merge branch A into beanch B? A and B will both contain the commits of A, but in B there may be commits of B between the commits that were merged. Do the same commits of A then have different parents depending on which branch they are on?
In git it's all just labels/pointers
Of course actual "branches" in the commit tree exist whether you label them or not. Until `git` does a garbage collection and gets rid of anything that doesn't have a pointer ultimately leading to it - something that a human would understand aka branch/tag. And that's why we call these labels "branches" as well but it's actually one word for two things here. The actual tree branch and the label that's called branch.
And a branch and a tag are basically the same exact thing underneath, just a file in the `.git` directory somewhere that contains a commit hash. All the meaning and differentiation of branch or tag is just in the human brain and how we and our tools treat them. Such as if you look at a particular commit in your tool of choice, it will tell you which branch it's part of. To create a branch you can literally just create a thousand randomly named files in the right part of the `.git` directory containing the same commit hash and suddenly this commit "is on all those branches". That's what git does and why creating a branch in git is so super fast.
Or labels and annotations.
Or something else.
Absolutely not. Commits are immutable (representing whole repo state, not a diff), and branches are just (mutable) pointers to them.
As the sibling already noted, a merge commit is just a regular commit. It simply points to multiple parents, "merging" them. Aside of the whole machinery to resolve conflicts etc. that's pretty much all there is to it.
When your graph topology allows it, you can also merge branches without generating a new commit (so called "fast forward" merges) - such a merge does nothing but rewrites the branch pointer. You can also create merge commits that point to more parents than two ("octopus" merges). Reconciling the commits' content can get quite complicated in such cases, but from the repo graph perspective it's nothing special.
To make things more clear: Repo state here is the contents of all files, and some metadata including a pointer to the previous commit.
So a commit hash uniquely identifies not only a set of files but the unique history leading up to it! That's why we some people like to call git the original block chain (there's no proof of work involved of course so it can never be used for payments or anything like that, but the merkle tree bit is similar enough).
1. Create a new merge commit with two parents: the commit pointed to by A and the commit pointed to by B.
2. Set branch B to point at the new merge commit.
This is a non-linear history; when comparing some commits there isn't a "before" or "after."
Other ordering is possible, e.g. --topo-order