Accessing elements of same id before and after scrolling the screen


My query is as below:
Let’s say I have about 8 drop downs in the screen including after scrolling up to the end of the screen.
Assume that only 4 elements are visible when the screen is loaded initially.
Please tell me how are these indexed (from 0 or from 1).
And, let’s say the screen is scrolled to middle and 4th element is visible and it is as first element.
How do we need to access it? with the index as against 0/1 or 3/4.
Hope I am able to make it understandable.

//Assume that below element is 4th initially and 1st element when screen is scrolled.


Please help with this.