I actually found out that we can mint an NFT for free using Rarible. Rarible offer users the option to use “lazy minting” to create NFTs free of cost. With this feature, the NFT is not created right away. It is only minted when it is bought and the fees are deducted from the purchase payment.
Mint are never free, there is always a fee to pay.
Lazy mint just means that you will mint when the gas fees are cheap.
As for opensea if you list an NFT there, it's not an NFT until someone buys it and pays the gas fees