In Ethereum ERC-721 or ERC1155, off-chain metadata can be saved on private storage like S3. In solana, I want to use SPL to mint NFT with metadata on own storage, it is returning type mismatch error. Is it possible to use own/private storage for off-chain metadata with SPL instead of metaplex?
Metaplex's Token Metadata Standard uses an "uri" field that points to a jsonFile. This json can be stored on any storage, like IPFS, AWS, Arweave, your own server, etc.
If you wanna mint some singles NFTs (instead of a collection) a fast and simple way is using Metaboss's mint command, that will create an on-chain NFT with the off-chain metadata (json) pointing to a storage. Metaboss is a Rust-CLI that has a lot of utilities for Solana-NFTs.
If you wanna create a NFT-collection I recommend you to use Metaplex's Candy Machine where you can use Arweave, IPFS, Pinata, NFT-Storage or AWS as storage, and you will be able to create the mint page so everyone can mint your project.
I hope this can solve your question, I will be looking if more questions appears through this one!.