Basically how does git merge know that a merge is in progress so that --abort or --continue makes sense? And how do it know that's not something else (fx rebase) in progress?

The reason I want to know is some curiosity, but also that I'm writing a custom command that includes a merge that can potentially conflict. So I think I should handle that scenario in a meaningful manner (and I don't think for the user to run git merge --continue/--abort then).