Depending on your platform there are many options to automate this process, for shopify you have oberlo, for woocommerce, you have alidropship or woocommerce dropshipping, for bigcommerce you also have dropshipping app.
And overall there are many tools that helps you automate this process with the help of chrome extensions and their software like importify, shopmaster and more.
For manual work,
for order processing, you have to provide the customer information in place of your information that you usually do in any ecommerce store and proceed to order.
Chrome extension also automate this fields data and you proceed to order.
If you are purchasing in bulk then best option is to contact supplier and follow through. Rather opt for Alibaba for wholesale.
Hope this answers you.