In .NET Core there is a nuget package you can add that generates the open api docs based on your controllers. It's a good start, though to make it fully accurate you have to add the right attributes to each endpoint. It works pretty well.

What I find really irksome about the .NET openapi tooling is the client generators use RestSharp under the hood which doesn't support HttpClient. There's no way you shouldn't be using HttpClient in 2020. So companies leveraging openapi to generate their SDKs are generating sub-par code.