How does Mui configure its exports without an `exports` property in the package.json?

In Mui's documentation here they recommend using

import Button from '@mui/material/Button';
import TextField from '@mui/material/TextField';

style imports instead of

import { Button, TextField } from '@mui/material';

The reason for this, as I understand it, is that that @mui/material style import will ultimately go through a barrel file - and bundlers like Webpack:

1. May not be able to treeshake accurately for production bundles
2. Will not treeshake for development bundles, giving slower development build times.

Webpack respects a exports property in the package.json (documentation here) to declare submodules accessed via @mui/material/Button style imports.

However, Mui doesn't actually appear to use these in their package.json

So how are these imports working?

It's standard node module resolution.

If you examine the rest of the directories/files in the npm package, you'll note that there's no src/lib/dist like directories. Note also there's no files property in the package.json

It's all

Accordion/
  index.js
AccordionActions/
  index.js
...
index.js
package.json

The root index.js is what's referenced in the main property of the package.json - this is the barrel file strategy used in the @mui/material style imports.

But the @mui/material/Button style imports are working just by referencing the directory and accessing the index file in it.

Mui's build process must involve building to a completely seperate directory and then copying over the package.json, as opposed to selectively choosing a folder to publish with the files property.