In which way do webhooks notify a client about server or application changes by default?

Asynchronous describes a processing model but not the timing characteristic of webhooks - the notification is sent immediately at the time of the event, not with arbitrary or scheduled delay.

Periodic notification describes polling, where the client checks for updates on a fixed schedule, which is the opposite of the server-initiated webhook model.

Webhooks are an event-driven mechanism where the server immediately sends an HTTP POST request to the client's configured endpoint the instant a specified event occurs, enabling real-time notification. This push model is the defining characteristic that differentiates webhooks from polling, where a client repeatedly requests updates on a schedule. The real-time delivery makes webhooks efficient for time-sensitive integrations and automation.

Synchronous means the client blocks waiting for a response within the same connection - webhooks are outbound server-initiated pushes, not responses within a synchronous request cycle.