"Merge 2 branches" is already far from being a primitive. A git repository is just a graph of snapshots of some files and directories that can be manipulated in various ways, and git itself is a bunch of tools to manipulate that graph, sometimes directly (plumbing) and sometimes in an opinionated way (porcelain). Merging is nothing but creating a node (commit) that has more than one parent (not necessarily two) combined with a pluggable tool that helps you reconcile the contents of these parents (which does not actually have to be used at all as the result does not have to be related to any of the parents).

(you may know that already, but maybe someone who reads this will find this helpful for forming a good mental model, as so many people lack one despite of working with git daily)