Website service host and backend, need help understanding and what to do

[GrimOfDoom]

I finished this course from udemy on beginner full stack web development, it used node.js and Mongodb for api control. I do not like MongoDB with how it handles the database- as one programmer's bad usage or manual insert means the database table itself could just die. I am not looking specifically for sql, but I would like to be able to use C# for API, as I have a large personal preference for that language (and I am much better at it than Javascript. I have personal gripes with how it handles stuff, blah blah more complaining. I want to pretty much reduce Javascript as much as I can, only for client side usage). Now, when I try to search around with c# API, I get a lot of articles and posts with cross origin resource sharing, and how it can cause problems and possibly security risks? I lack knowledge in this part.

 

But what I am also confused a bit with, is how I can send post/get/put/etc requests through a normal website (say serviced through nginx, the only thing I know because the course I did), but send the request to the second running C# program/service that wait's for API calls. What I am understanding is the user gets webpage, and then it has buttons to perform actions. How does the actions work in getting api responses since the api is hosted in the cloud, but there is no direct link. 

 

(This course i fully did, had way too little instructing on the need to knows with the language- but not exactly how things play together.  I did not know react was a one page webapp library until I finished the course and went to the website to look around more)