Clarifying OpenAPI callback spec - callback

Question

I'm reading the OpenAPI callback spec, my main sources are
https://spec.openapis.org/oas/latest.html#callback-object (spec)
https://github.com/OAI/OpenAPI-Specification/blob/main/examples/v3.0/callback-example.yaml (example)
The spec (first link) states that the structure should be like:
Operation Object
callbacks: Callback Object
[pattern]: Path Item Object
get: Operation Object
post: Operation Object
etc...
However the example (second link) describes a callback-enabled operation like so (ignored the irrelevant parts):
openapi: 3.0.0
paths:
/streams:
# Code above is irrelevant, just want to keep the structure
post: # Operation Object level
callbacks: # Callbacks Object
onData: # Not sure what this is???
'{$request.query.callbackUrl}/data': # Path Item Object level
post: # Operation Object level
...
My question is where does eiter onData or this: '{$request.query.callbackUrl}/data' come from? According to the spec the Path Item Object should be one level above, and the onData (or the interpolated URL) field has no business being there, one level of nesting should be removed.
According to my interpretation, the example should either look like this if we are following the spec:
openapi: 3.0.0
paths:
/streams:
post: # Operation Object level
callbacks: # Callbacks Object
'{$request.query.callbackUrl}/data': # Path Item Object level
post: # Operation Object level
...
Or the spec should be like this if we are following the example:
Operation Object
callbacks: Callback Object
[name]: Paths Item Object
[pattern]: Path Item Object
get: Operation Object
post: Operation Object
etc...

Answer 1

Operation-level callbacks field is a map where the keys are arbitrary IDs/names and the values are Callback Objects. onData in this example is a key/name/ID of the callback.
Each Callback Object is also a map where the keys are runtime expressions that evaluate to the request URL for the callback, and the values are Path Item Objects that define the callback request format (HTTP method, request body, expected status codes, etc.)
paths:
/streams:
post:
...
callbacks:
# Arbitrary name/ID of this callback
onData:
# Callback Object starts here
# This expression defines the URL to send the callback to
'{$request.query.callbackUrl}/data':
# Path Item Object starts here
post:
...
Here, {$request.query.callbackUrl}/data means that the callback URL is constructed from the callbackUrl query parameter of the parent request, with /data appended to it. For example, if a developer sends a callback subscription request as
POST /streams?callbackUrl=https://myserver.com/webhooks
the callback will be sent to:
https://myserver.com/webhooks/data
Check out the Callbacks guide on swagger.io for additional explanations.