WebClient uses an asynchronous, non-blocking solution provided by the Spring Reactive framework.

While RestTemplate uses the caller thread for each event (HTTP call), WebClient will create something like a “task” for each event. Behind the scenes, the Reactive framework will queue those “tasks” and execute them only when the appropriate response is available.

The Reactive framework uses an event-driven architecture. It provides means to compose asynchronous logic through the Reactive Streams API. As a result, the reactive approach can process more logic while using fewer threads and system resources, compared to the synchronous/blocking method.

WebClient is part of the Spring WebFlux library

https://www.baeldung.com/spring-webclient-resttemplate