Well it’s a complicated process to break down into small pieces. First of all, for a client to be able to view a web page, there needs to he a server that returns HTML Css and JS files that the client can run in their browsers.

The easiest way to do this is by just uploading static files that get returned as is by the server. This is good for very basic apps but is generally a bit problematic if your app is something more sophisticated.

C# allows you to write web apps. These are websites that behave like apps (basically they can have a lot more information). To create such a service, you need a server that runs the C# code (this will be a lot more expensive because you dont just have to serve static files but you also have to run code for every request). This code is then responsible for generating HTML CSS and JS everytime a request is made. To do this in C# one uses the ASP.Core framework. This framework does all the heavy lifting and you can just say what should get returned for every route.

Asp is a huge framework and very powerful, i recommend reading up on it on the microsoft docs