Can somebody explain to me how Routing works in asp.net core? I tried reading the docs and tutorials but they all don't explain very Well what's going on. It's just "copy this and that piece of code..." with magic keywords All over the place.

Also I don't understand what those project templates for SPAs are supposed to be? Why is the Server and Client Code mixed? As far as I understood it there is an API which gives me jsons, and the react / angular/vue Client which use the API. Aren't those two separate things? And why is there MVC and Razer pages Code in the API side?