To select an element in one of the arrays you have this...
_array = [ 33,79, [s4,20,1], 25]
_a = (_array select 2) select 1
Select 2 means you are selecting the 3 element in the whole array
That's [s4,20,1]
select 1 is the second element in that array,
so _a = 20
other example
_array = [ [1,"hello"], [L4,20,1], 25]
_b = (_array select 0) select 1
select 0 is the array [1,"hello"]
select 1 is the second element in that array
_b = "hello"