Both Appium and Selenium are best in their own way and possess different features. In order to automate any web application, Selenium is best while to automate any mobile application, Appium is best.
No Appium is not the extension of Selenium. Appium uses Selenium’s JSON wire protocol extension to support mobile-web, native and hybrid applications.
Appium does not require any coding knowledge. But having a basic knowledge of programming languages will be beneficial.
Appium is easy to learn. If you are already familiar with the Selenium tool, you will just take a couple of hours to learn Appium.