The C standard defines the [] operator as follows:
a[b] == *(a + b)
Therefore a[5] will evaluate to:
*(a + 5)
and 5[a] will evaluate to:
*(5 + a)
a is a pointer to the first element of the array. a[5] is the value that’s 5 elements further from a, which is the same as *(a + 5), and from elementary school math we know those are equal (addition is commutative).