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.