I believe that to be Copy & Paste. It's so humble and well ingrained in our experience that we no longer think of it as code or as a tool, yet it's what ties together most data flows in our digital environments, at least for end users who have no access to APIs. Before the Object-Verb syntax of copy-paste was invented (i.e. "select object+copy+select destination+paste"), there were only Verb+Object commands ("move+select object+select destination"), and it sucked.

Cut-Copy-Paste has been for decades the single tool that allowed end users to create their own workflows, combining several applications to carry out the successive steps of a single task not put together by a developer. Copy/paste is THE universal tool that is almost guaranteed to work in transferring information between arbitrary tools, to the point that it feels broken or artificially restricted when it's not available.

Nowadays it gets competition from intents (the "share" button) in mobile OS and the web, but copy/paste is still more flexible. And it's somewhat similar to the role of pipes in the shell, but those are our of reach for end users.