Answer 1

if list[4]=5

list[list[4]] is the same as
list[5]
and list[5]=6

Answer 2

inner list: list[4] is 5. (index of outer list)
outer list: list[5] is 6.

Answer 3

Ben Rückert

= list[list[4]]

at index 4 value is 5 so list[4] = 5

= list[5] // now here index will be 5
= 6

Answer 4

understanding that list[4]=5
and that list[5]=6
just replace 5 with list[4]
and you get
list[list[4]]=list[5]=6

Answer 5

Ben Rückert

normally you have this: list[index]
list[0], list[1] and so on

but in your case the index is also a value from the list
index = list[4] = 5
list[index] = list[5] = 6