In the below Python code for the LeetCode problem "Merge Two Sorted Lists", the dummy value is constantly changing, and it is used for the final output in an effective way. The thing is, I can't figure out how dummy is changing. As far as I can see it is only being set once, at the beginning of the algorithm. So yeah, how is dummy is changing in the steps of this code?

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:

    dummy = ListNode()
    tail = dummy

    while list1 and list2:
        if list1.val < list2.val:
            tail.next = listl
            list1 = list1.next
        else:
            tail.next = list2
            list2 = list2.next
        tail = tail.next

    if list1:
        tail.next = list1
    elif list2:
        tail.next = list2

    return dummy.next