"git checkout <commit id>" is changing branch to "no branch"

Is that commit in the other branch? Git checkout <commitid> will just switch over to the other branch if the commit has happened in the other branch. You will want to merge the changes to your first branch if you want the code there.

If you are branch master and you do a git checkout <SHA>

I'm fairly certain that this causes git to load that commit in a detached state, changing you out of the current branch.

If you want to make changes you can and then you can do a git checkout -b <mynewbranch> to create a new branch based off that commit and any changes you have made.

If you checkout a commit sha directly, it puts you into a "detached head" state, which basically just means that the current sha that your working copy has checked out, doesn't have a branch pointing at it.

If you haven't made any commits yet, you can leave detached head state by simply checking out whichever branch you were on before checking out the commit sha:

git checkout <branch>

If you did make commits while you were in the detached head state, you can save your work by simply attaching a branch before or while you leave detached head state:

# Checkout a new branch at current detached head state:
git checkout -b newBranch

You can read more about detached head state at the official Linux Kernel Git docs for checkout.

By checking out to one of the commits in the history you are moving your git into so called 'detached state', which looks like is not what you want. Use this single command to create a new branch on one of the commits from the history:

git checkout -b <new_branch_name> <SHA1>