I would like to know if it is possible to create an NFT without going through an online platform such as Opensea, with only a wallet for example And how to do it if possible
Best regards
Of course you can create nfts without the help of nft marketplaces such as opensea, foundation app etc.
For that, you need to write an erc721 standard contract and you can do it by importing the openzeppelin erc721 contract.
Then deploy the contract in the blockchain.
You can follow the instructions to create and mint nfts.
Create NFTs Using Solidity With Open Zeppelin and Publish It on IPFS