Short Answer: When You receive an order on Your Pre-Built Shopify Store, You just need to fulfill it through Your fulfilment app and Your supplier will handle the rest.
How to Fulfil Orders from Your Pre-Built Shopify Store:
1) Your client places order
2) You submit the order through Your fulfilment app and pay the product cost (e.g. Dsers, Oberlo)
3) Your supplier receives order
4) Your supplier ships order (usually within 24-48 hours)
5) You fulfil order in Your Shopify fulfilment app (e.g. Dsers, Oberlo)
6) Tracking information is automatically sent to client via email.
7) Order is delivered to customer within 7-14 days via USPS/UPS.