Can anyone explain me how the below code works

Below code works for me and i want to know how its working. The code is actually getting the country code based on the ip address and loading it into the input box.

My doubts are:

1. loadJSONP() function loads the script but why they are mentioning callback string as query paramater.
2. lookup() function how they are getting the value from the script url that was loaded previously and removed

import React from 'react';
import ReactDOM from 'react-dom';
import IntlTelInput from 'react-intl-tel-input';
import 'react-intl-tel-input/dist/main.css';

const loadJSONP = (url, callback) => {
  const ref = window.document.getElementsByTagName('script')[0];
  const script = window.document.createElement('script');
  script.src = `${url + (url.indexOf('?') + 1 ? '&' : '?')}callback=${callback}`;
  ref.parentNode.insertBefore(script, ref);
  script.onload = () => {
    script.remove();
  };
};

const lookup = (callback) => {
  loadJSONP('http://ipinfo.io', 'sendBack');
  window.sendBack = (resp) => {
    const countryCode = (resp && resp.country) ? resp.country : '';
    callback(countryCode);
  }
};

ReactDOM.render(
  <IntlTelInput 
    defaultCountry="auto"
    geoIpLookup={lookup}
    css={['intl-tel-input', 'form-control']}
  />, 
  document.getElementById('root'),
);