How is `@babel/runtime` implemented so that it can reference dependency outside `node_modules`?

Question

From these lines on GitHub, they're referencing the regenerator runtime:

    "./helpers/regeneratorRuntime": [
      {
        "node": "./helpers/regeneratorRuntime.js",
        "import": "./helpers/esm/regeneratorRuntime.js",
        "default": "./helpers/regeneratorRuntime.js"
      },
      "./helpers/regeneratorRuntime.js"
    ],
    "./helpers/esm/regeneratorRuntime": "./helpers/esm/regeneratorRuntime.js",

but if you see the source code folder there is nowhere you can find the files ./helpers/regeneratorRuntime.js or ./helpers/esm/regeneratorRuntime.js.

So how is this be done?