[Question] Where is the schema-based serialisation

[WoodyWoodsta]

The docs say:

Roarr is opinionated about how it serializes (converts objects to JSON string) log messages, e.g. in Node.js it uses a schema based serializer, which is very fast, but does not allow custom top-level properties.

Having had a root around the source code, I can't find where this schema-based serialisation is occuring. I'd like to implement my own serialiser to log the levels in their human-readable names (which believe it or not are also machine-readable and more commonly used) but I'd like to make sure I'm following the current serialisation implementation as closely as possible.

Could you point me in the direction of where the message is being serialised by schema?