I'm struggling to follow why a merge base makes a difference? Or even exactly what one is?

A couple of questions which might help clear things up:

Do you consider `main` a branch?

If you create a new branch called `feature1`, do you consider the commits from before the branch from `main` to be "part of" that branch or not?

What if you delete `main`? Are the commits from before the branch part of the `feature1` branch then? What if there are multiple surviving feature branches that share varying amounts of common history?