How to automate the shopify to other online store process?

I am pretty new to this stuff but I have been learning a lot about it, from what I've learned you find a product from something like aliexpress or ebay and put it on your site and when someone orders something you go to that site and order it to their address. If this is correct what happens when your getting thousands of orders and you can't possibly fill out all the information? Are there any apps or ways to automate this process? Any help would be appreciated, thanks.

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