When I recently had to look into some of the specifics of using Swagger and OpenAPI for designing an API specification and generating a Spring Boot application skeleton from that specification (using OpenAPI Generator) I came across this very useful and comprehensive article on the subject on the Reflectoring blog:

API-First Development with Spring Boot and Swagger

From creating an API specification with the Swagger editor to generating code (with the OpenAPI Generator Maven plugin, specifically) and implementing the generated interfaces the article covers everything you need to know for getting started with code generation based on OpenAPI.

As is usual with Reflectoring blog posts, there are working code examples available on GitHub.