There are tons of resources available to learn more about API's in ASP.Net core. Some of the most up-to-date is the Microsoft documentation itself. To learn more and brush up on your C# API skills check out the following resources below.
- Creating Web API's with ASP.Net Core
- Tutorial: Create a web API with ASP.Net Core
- Microsoft Learn: Creating an ASP.Net Core API
If you have found helpful tutorials on your own, feel free to let me know!