```python
from pm4py.objects.process_tree.obj import ProcessTree, Operator

# Create the root node (parallel execution of order and payment)
root = ProcessTree(operator=Operator.PARALLEL)

# Create the order branch
order_branch = ProcessTree(operator=Operator.SEQUENCE, parent=root)

# Create the activities within the order branch
create_purchase_requisition = ProcessTree(label="Create Purchase Requisition", parent=order_branch)
approve_purchase_requisition = ProcessTree(label="Approve Purchase Requisition", parent=order_branch)
create_purchase_order = ProcessTree(label="Create Purchase Order", parent=order_branch)
receive_goods = ProcessTree(label="Receive Goods", parent=order_branch)

# Add the activities to the order branch
order_branch.children.append(create_purchase_requisition)
order_branch.children.append(approve_purchase_requisition)
order_branch.children.append(create_purchase_order)
order_branch.children.append(receive_goods)

# Create the payment branch
payment_branch = ProcessTree(operator=Operator.SEQUENCE, parent=root)

# Create the activities within the payment branch
receive_invoice = ProcessTree(label="Receive Invoice", parent=payment_branch)
approve_invoice = ProcessTree(label="Approve Invoice", parent=payment_branch)
make_payment = ProcessTree(label="Make Payment", parent=payment_branch)

# Add the activities to the payment branch
payment_branch.children.append(receive_invoice)
payment_branch.children.append(approve_invoice)
payment_branch.children.append(make_payment)

# Add the branches to the root node
root.children.append(order_branch)
root.children.append(payment_branch)

# The process tree is now complete and stored in the 'root' variable
# You can now use this process tree for further analysis or visualization
```

**Explanation:**

1. **Parallel Root:** The process starts with a parallel gateway (`Operator.PARALLEL`) represented by the `root` node. This allows the order and payment processes to happen concurrently.
2. **Order Branch (Sequence):** The `order_branch` represents the sequential steps involved in ordering goods. Each activity within this branch is a child of `order_branch` and is executed in sequence.
3. **Payment Branch (Sequence):** Similar to the order branch, the `payment_branch` outlines the sequential steps in processing payments.
4. **Activities (Leaf Nodes):** Each activity is represented by a `ProcessTree` object with a `label` attribute specifying the activity name. These are the leaf nodes of the process tree.
5. **Parent-Child Relationships:** The code explicitly sets the `parent` attribute of each child node to its respective parent, establishing the hierarchy of the process tree.

This code creates a process tree for a basic Purchase-to-Pay process. You can expand this further by adding more complex scenarios like handling exceptions or incorporating loops for iterative tasks. Remember to adjust the activities and structure to accurately reflect your specific Purchase-to-Pay process.
