OP_RETURN is just the standard way of adding a small arbitrary data to the bitcoin blockchain, which is why they call it "nulldata".
For additional reference, OP_RETURN used by OmniLayer and Runes protocol to create token. OmniLayer was mainly used for USDT which AFAIK no longer supported by Tether, while Runes is about NFT/token.
That's right but it is worth mentioning that from the perspective of Bitcoin protocol and its smart contract language (scripts) what is followed by OP_RETURN is arbitrary data that won't be interpreted in any other way.
Outside Bitcoin protocol, usually in centralized databases (like Tether) they can interpret that arbitrary data however they like with their own protocol such as using it as token transfers inside Omni Layer.
OP_RETURN, in Bitcoin protocol, prune transaction embedded with data out of the Utxo set and make them unspendable, which creates a way for the Opcode to be used for burning bicoin. Runes protocol, for instance, then uses the Opcode to hold just about 80bytes of fungible data token, that includes, name, minting, token ID, and symbol, to aid the transfers of Runestoken.