FirstRow = Selection.Offset(1, 0).Row

Question

I'm trying to understand what's going on in the below code.
I understand that the first line is defining the last row by searching for the last cell within a column that has something written in it.

When I see the script working it does not go from bottom up but rather tries to find the last row that has data in it in column "S" and then jumps to do the tasks one by one in the following rows copying data into Columns S, T, U. But I don't really understand how this line of code represents what I see...

LastRow = Wb.Range("A1").End(xlDown).Row

For i = LastRow To 1 Step -1
   Range("S" & i).Select
   If Range("S" & i).Value <> "" Then
        FirstRow = Selection.Offset(1, 0).Row
        Exit For