How do I assign Prefab in the Assets to GameObject though code?

Questions & Answers
1

Is it real to do it without assigning in inspector?

2

If you have your Prefabs in a folder located in Assets>Resources, then you can load them using Resources.Load and Instantiate them through code during Play.

Creating new prefabs is done via Editor however. What specifically are you trying to achieve? Unity - Scripting API: Resources.Load