Lately I’ve discovered that actions/checkout@v4 in GitHub doesn’t behave as I thought. I have a very simple pipeline with first step defined like this:

on:
  pull_request:
    branches:
      - 'main'

jobs:
  run-tests:
    runs-on: 'ubuntu-latest'
    defaults:
      run:
        working-directory: ./
    steps:
      - name: Checkout
        uses: actions/checkout@v4
      - runing tests etc etd

My idea was that git just checks out on my branch. Turns out it takes my commit and merges them on main. I’ve started looking at exact command that are run:

1. git init /home/runner/work/my-repo/my-repo

2. git remote add origin https://github.com/MyOrg/my-repo

3. git config --local gc.auto 0

4. …some commands to set up auth

   git config --local --name-only --get-regexp core.sshCommand
   git submodule foreach --recursive sh -c "git config --local --name-only --get-regexp 'core.sshCommand' && git config --local --unset-all 'core.sshCommand' || :"
   git config --local --name-only --get-regexp http.https://github.com/.extraheader
   git submodule foreach --recursive sh -c "git config --local --name-only --get-regexp 'http.https://github.com/.extraheader' && git config --local --unset-all 'http.https://github.com/.extraheader' 
   

5. Fetching

   git -c protocol.version=2 fetch --no-tags --prune --no-recurse-submodules --depth=1 origin +branchRemoteRef:refs/remotes/pull/69420/merge
   

6. Some config

   git sparse-checkout disable
   git config --local --unset-all extensions.worktreeConfig
   

7. Checkout

   git checkout --progress --force refs/remotes/pull/69420/merge
   

And once 7. is run I can see this message:
HEAD is now at #someCommitHash Merge branchRemoteRef into #latestMainCommit.

So I don’t think anything important (for my case) happens in other steps than 5 and 7 but I just pasted everything to be clear. I feel like I’m missing some git knowledge. Why, when git checkout is run, it merges into main? How does it know what is the latest commit on main? How does it even know that it should take from main (does it just use default branch)? I’ve read documentation and tried to search online but couldn’t find an answer. This post /a/70274029 confirms that this magic merging happens but I would like to understand why it happens.