Branches are names that move through a graph

First, build a mental model

A commit records a snapshot and parent links. A branch is a movable name pointing at a commit. HEAD normally identifies the checked-out branch; detached HEAD points directly at a commit.

1. Creating a branch at the current commit creates another name at that position, not a full copy of every file.
2. New commits advance the checked-out branch; other branch names stay where they were.
3. A fast-forward moves a branch name to a descendant when no divergent merge is required.
4. A merge of divergent histories usually creates a commit with both histories as parents, after content is reconciled.
5. Fetch updates remote-tracking information without automatically merging it into the checked-out branch. Inspect ancestry before choosing the next operation.

# Start with main pointing at a known commit.
git switch -c feature/score-label
# Make one focused change, inspect it and commit it.
git log --oneline --decorate --all
git switch main
# If main has not moved and is an ancestor of the feature:
git merge --ff-only feature/score-label
# --ff-only refuses if a fast-forward is not possible.

1. New branch

   main and feature initially name the same commit.

2. Feature commit

   Only feature advances while it is checked out.

3. Fast-forward

   If ancestry permits it, main can move to the feature commit without creating a merge commit.