Marketplaces such as Opensea are like Ebay, they are a platform for people to resale their NFTs. The majority of NFT collections first do what is called an "initial sale". An initial sale happens from the collection website where people can buy one or many NFTs, also called minting. Once an NFT is minted, it will automatically appear on marketplaces like Opensea where you can choose to resale your NFTs.In order to do an initial sale, you have to first deploy a smart contract. A smart contract is required because this is where the NFT ownership is stored and can prove you are the owner of which NFT.Your alternative is to upload your entire collection to Opensea manually (it cannot be automated) or deploy a smart contract and airdrop the entire collection to yourself (costly).