Umbraco's request pipeline is the process of building up a URL, resolving the requests, and returning correct content.
The inbound process is triggered by UmbracoRouteValueTransformer and then handled with the Published router. The Published Content Request Preparation process kicks in and creates a PublishedRequestBuilder which will be used to create a PublishedContentRequest.
What it does:
It ensures Umbraco is ready, and the request is a document request.
Ensures there's content in the published cache, if there isn't it routes to the RenderNoContentController which displays the no content page you see when running a fresh install.
Creates a published request builder.
Routes the request with the request builder using the PublishedRouter.RouteRequestAsync(…).
This will handle redirects, find domain, template, published content and so on.
Build the final IPublishedRequest.
Sets the routed request in the Umbraco context, so it will be available to the controller.
Create the route values with the UmbracoRouteValuesFactory.
This is what routes your request to the correct controller and action, and allows you to hijack routes.
Set the route values to the http context.
Handles posted form data.
Returns the route values to netcore so it routes your request correctly.
When finding published content the PublishedRouter will first check if the PublishedRequestBuilder already has content, if it doesn't the content finders will kick in. For more information, see the Find published content section in the Published Content Request Preparation article.
This information is also used during the Controller & Action selection process.