It’s what the entire internet is built upon. it has nothing to do with Unity. You can make calls from Unity using the UnityWebRequest class.
https://www.codecademy.com/article/what-is-rest
Hie thee to a good tutorial and get familiar with the basic process or you are going to struggle for a very long time trying to make a square peg fit into a hole that doesn’t even exist. 
Once you have your web service up and are ready to make REST calls from API, here’s how to do it:
Networking, UnityWebRequest, WWW, Postman, curl, WebAPI, etc:
https://discussions.unity.com/t/831681/2
https://discussions.unity.com/t/837672/2
And setting up a proxy can be very helpful too, in order to compare traffic:
https://support.unity.com/hc/en-us/articles/115002917683-Using-Charles-Proxy-with-Unity