In my previous articles listed below, I have been shown how to use Swagger, especially the Springdoc implementation. for doing the code first/bottom-up approach.

OpenAPI 3 Documentation With Spring Boot 
Doing More With Springdoc-OpenAPI
Extending Swagger and Spring Doc Open API

This time I am writing about the design first/top-down approach. I am not writing about the usual generated Java server, and say, associated Angular TypeScript client code; but first, some background context.