How to create a scene programaticly

YAML, XML, XAML, JSON or other are all the same thing, a text file just like a plain notepad file.

Only difference is that the extension is different so that it is recognized by software to contain special keyword or layout to speed up parsing.

Most engine or framework come up with their own extension and their own layout, for example Marmelade uses .mkb for prefab, but it is just a text file with special character to indicate beginning and end of file and so on.

Then you can come up with your own text file for your scene and your own method to read and parse your file. This below is a .fafase extension

 {
    scene : 01_
    scenename : FirstScene_
    object : 
        @ 
          name : Player_
          prefab : Player_
          position : 00.00,00.00,00.00_
          rotation : 00.00,00.00,00.00_
          scale : 00.00,00.00,00.00_
 }

Now you can come up with a parsing system that I will simplify greatly:

 void CreateFromFafaseFile(string fafaseFile){
     // Get scene number and scene name and compare to current scene
     while(@ is found)
        // start parsing info using _ as end of line
        // Get the prefab and fetch it with Resources.Load and put it in prefab
        // Get the name and store it in string name
        // Get the position , rotation and scale and store in V3
        // instantiate using parsed info
        GameObject obj = (GameObject)Instantiate(prefab, position, rotation);
        obj.transform.scale = scale;
        obj.name = name;
        //get to next @ character
 }