Im a beginner in developing CorDapps, so far I have successfully written flows and such, I am currently learning how to code APIs for Corda, and I'm not sure if I could test Corda APIs in postman like regular APIs, any info would be greatly appreciated.
The Corda Webserver that you're referring to is simply a Jetty server that connects to the Corda node using the CordaRPCClient library, then provides an HTTP API that allows the webserver to map HTTP requests from users to RPC operations on the node. You can test this API in the same way as a regular server (e.g. by using Postman).
Please note that the Corda Webserver is deprecated as of Corda 3 and you are expected to create your own Java webserver mapping HTTP requests to RPC operations instead. See the Spring Webserver sample here for an example.