Hi,
How is the best process to do internal warehouse transfer stock?
Normally using internal shipment.
Or need to do customer shipment then suplier shipment to do this process?
You must use internal shipment. Other shipments will involve supplier or customer location which mean that the company will no more own the product.
You can also define a location lead time between two warehouses. In this case the internal shipment will use a transit location to represent the travel time.