How to customize the graphql query in strapi v4?

this article example is not clear to me to understand the customize graphql . Would you please give me a example of query an mutation on graphql customization on strapi v4.

System Information
  • Strapi Version: 4.0.2
  • Operating System: Ubuntu 20
  • Database: MySql
  • Node Version: 14.17
  • NPM Version: 6.0.0
  • Yarn Version: