Components of the hosting

The .NET Core default hosting setup consists of several components which are described here.

Entry-point hosts

.NET Core comes with several executables/libraries which act as the main entry-point to start code execution. These are typically referred to as the "host":

dotnet (executable) - which comes from a shared location and is typically the latest version available on the machine. This is also sometimes called the "muxer".
apphost (executable) - which is used to give app an actual executable which can be run directly. The executable will be named using the application name. The advantage of having an app-local executable is that it can be customized for each app (not just the name, but icon, OS behavior and so on).
comhost (library) - which is used to enable COM server hosting. Component which wants to expose COM server objects will be built with this dynamic library in its output. The comhost then acts as the main entry point for the OS.
ijwhost (library) - which is used to enable loading of IJW assemblies. The library exposes functionality needed by the C++ compiler to generate mixed mode assemblies.
nethost (library) - which is used by native apps (any app which is not .NET Core) to load .NET Core code dynamically.

The entry-point typically does just one thing: it finds the hostfxr library and passes control to it. It also exposes the right entry points for its purpose (so the "main" for dotnet and apphost, the COM exports for comhost and so on).

dotnet host - hostfxr is obtained from the ./host/fxr/<highestversion> folder (relative to the location of the dotnet host).
apphost, comhost and the others - hostfxr is located using this process:
1. The app's folder is searched first. This is either the folder where the entry-point host lives or in case of apphost it is the path it has embedded in it as the app path.
2. If the DOTNET_ROOT environment variable is defined, that path is searched
3. The default shared locations are searched

Host FXR

This library finds and resolves the runtime and all the frameworks the app needs. Then it loads the hostpolicy library and transfers control to it.

The host FXR library reads the .runtimeconfig.json of the app (and all it's dependent frameworks) and resolves the frameworks. It implements the algorithm for framework resolution as described in SharedFX Lookup and in Framework version resolution.

In most cases the latest available version of hostfxr is used. Self-contained apps use hostfxr from the app folder.

The main reason to split the entry-point host and the hostfxr is to allow for servicing the logic in hostfxr without the need to stop all instances of the executable host currently running. In the case of apphost and all the library hosts, servicing the entry-point host is impossible as they are part of the application itself.

Host Policy

The host policy library implements all the policies to actually load the runtime, apply configuration, resolve all app's dependencies and calls the runtime to run the app or load the required component.

The host policy library lives in the runtime folder and is versioned alongside it. Which version is used is specified by the app as it specifies which version of the .NET runtime to use (done directly or indirectly by referencing the Microsoft.NETCore.App framework, or carrying everything app-local).

Host policy library reads the .deps.json file of the app (and the .deps.json of all the referenced frameworks). It resolves all the assemblies specified in the .deps.json for the app and frameworks and creates a list of assembly paths (also called TPA). It does a similar thing for native dependencies as well.

Finally the host policy library loads the runtime coreclr library and initializes it (among other things with the TPA). The version of the runtime (and its location) is now already determined since the host policy was loaded from the same framework. Then it calls the runtime with the configuration information which runs the app or performs other requested actions (like COM activation).