Which is best software for extension development?

I am beginner at extension development so pls suggest me best software for extension development and pls tell me how to use that software.

Since you’re a beginner, I suggest you use Kodular’s IDE (ide.kodular.io).
The IDE handles all the ant build files and libraries so that you don’t have to worry about them.

However, if you’re looking for an offline variant/more flexibility, then any Java editor will work. Do note that you’ll have to setup App Inventor sources correctly before you can use your own editor.

Here’s a nifty guide you can follow: