Great question.

So far, I've seen people have been using in-line schema definitions within the asyncapi spec file itself. The spec can be version controlled so that teams can control the schema evolution.

However, the spec provides a way to refer an external schema file.

https://www.asyncapi.com/docs/specifications/v2.2.0#externalDocumentationObject

That way, you can host the schemas in a central location and point to them from the spec files. This will promote a centrailized schema governance, which is great when there are many teams sharing the same schema.

Hope I answered your question.