Now you will get it.

Recap – Function that calls itself.

How does Recursion actually solves a problem –
Let’s see how to Flatten the tree-

Input: root = [1, 2, 5, 3, 4, null, 6]
Output: [1, null, 2, null, 3, null, 4, null, 5, null, 6]

Here we need to move the left subtree to the right left in place (in the same tree) so it forms a linked list such that it maintains preorder traversal of binary tree.

Following are the 2 different code for this
Method – 1 : Top-Down Approach

 # Have to modify in place
    def flatten(self, root: Optional[TreeNode]) -> None:
        # To keep track of previous element
        prev = None
        
        def flatten_helper(root):
            nonlocal prev
            if root == None:
                return
          
            right = root.right
            if prev:
                prev.right = root
                prev.left = None
            prev = root

            flatten_helper(root.left)
            flatten_helper(right)

        return flatten_helper(root)

Below Explain it in action (Recommended speed is 1.5x)