OpenAPI and external services
Many organizations that are developing and sharing RESTful APIs will now provide a way of retrieving the information relating to the API in the OpenAPI specification. The OpenAPI initiative is a collective of individuals and organizations that have worked to define a standard for allowing APIs to be shared and standardized between platforms. Salesforce is a member of the initiative alongside the likes of Google, the UK government, Postman, eBay, and many more.
If the system you are integrating provides its API in an OpenAPI format, you can use the External Services feature within the Salesforce setup menu. Once you have loaded your file, Salesforce will generate a series of Apex wrapper classes for the different operations defined by the API, which you can then utilize directly in your code.
When working with an external system, should the API owner provide an OpenAPI specification, I would strongly recommend importing it to create an external service...
