I can create a collection on opensea and upload every item on it and sell without any code. But i can write smart contracts code and deploy the smart contract then list the address on opensea as well.
I want to know what's the difference. Hope someone can help me. Thanks!
Hi, welcome to the community! 
Maybe if you list the address, it can upload all items automatically, I am not sure for this, and I think you can ask the developer of the opeansea to get a more correct answer.