Shouldn’t the elements inside div be on the same line vertically ? Why have ‘two’ and ‘three’ text more inside than ‘one’ text (outermost div tag) ? This thing is demonstrated at 1 hr 7 min mark. Please clarify.
Div tag orientation
No, they are not on the same line vertically as the divs are nested one inside another and the some margin is provided to each box, which gives the orientation of elements shown.
It is very well explained in the video. 
Yes, that was covered in later part of video. Why doesn’t this apply to nesting of
(paragraph) tags , theres no effect of margin-left and margin-right ? what’s the reason?
Each tag has its own property and behaves accordingly. One can learn this with practice only or you can always refer the official documentation on MDN.