Career Site is an auto-generated feature once you set up your workspace domain properly. It gets updated dynamically once you publish a job on your career site in creating a job process.
It's a tier-based feature so can you find more information about it here.
To customize your career site you only need to go to your setting on the left side bar > Got to Company Profile and tap into gear icon on the top right of the page indicated as Career Page.
You can find here other qpage section to understanding what is it and how you can use them