1. Ordinals protocol: Each bitcoin is composed of 100 million satoshis. The Ordinals protocol allows each satoshi to be identified and traded through additional data. In fact, the order in which each statoshi is minted is used as a mark, and then the transaction inscribes the data into the witness data in the block. After inscription, the first statoshi can be used as a unique mark (determined by transaction hash or statoshi serial number). This can be used as an NFT on the BTC chain.

2. BRC20: A protocol for issuing tokens using the ordinals protocol. In fact, when writing witness data, a string of json is written, and this json follows a series of rules agreed upon by the brc20 protocol. For example, ticker name, maximum issuance, depoly/mint/transfer operations, etc.

3. Runes: A BTC token issuance protocol similar to BRC20. However, the data is not written into the witness data, but is implemented using BTC's OP_RETURN opcode, which directly adds the data to the transaction (the OP_RETURN opcode is limited to 80 bytes, so it is not suitable for NFT, but it is more than enough for tokens).