The documentation makes us believe that this example:

module.exports = {
  //...
  externals: {
    jquery: 'jQuery'
  }
};

will result in jquery imports to be replaced with jQuery variable:

In other words, when a string is provided it will be treated as root (defined above and below).

root: The library should be available as a global variable (e.g. via a script tag).

However it's not exactly true.
The default type of externals is defined by externalsType option. Which is by default defined as follows:

https://github.com/webpack/webpack/blob/master/lib/config/defaults.js#L189-L198

So for example in case of commonjs library target import $ from 'jquery'; will be transformed to require("jQuery").