There are various competing standards for describing JSON schemas. Some use separate JSON files, others put details inside the JSON files themselves. None of them are fully standardised the way JSON or XSD are, though.

I've seen https://json-schema.org/ in use most often, but it's not directly associated with JSON and support depends on how close your software stack seems to be to Javascript frontend web development. It's still an IETF draft at the moment, and development seems to have stopped: https://datatracker.ietf.org/doc/html/draft-bhutton-json-sch...

Alternatively, you could theoretically use YAML schemas to validate JSON, as standard JSON is also valid YAML.