[Python] MergeSort without merge function,, How does this algorithm merges the lists

def mergeSort(A):
if len(A) < 2:
return A

mid = int(len(A)/2)
left = mergeSort(A[:mid])
right = mergeSort(A[mid:])

r,l = 0,0
B = []

while len(B) < len(A):
if r >= len(right) or (l < mid and left[l] < right[r]):
B.append(left[l])
l += 1
elif r < len(right):
B.append(right[r])
r += 1

return B

print(mergeSort([4,3,6,9,8,5,1]))


My doubt in the above program is how the lists are merged without a separate merge function?

After the recursive function calls in the end I think left and right lists contain only one element each... they are sorted and inserted in to list B using a while loop... what about other elements.. because here while loop is executed only once and how the list A is again split and merged...I got this program in internet and it works perfectly fine.

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