host and app config, appsettings Urls takes precedence over ASPNETCORE /2

[Rick-Anderson]

Fixes #25626
Internal review URL

@Hoffs @chertby please review.

[Rick-Anderson]

@serpent5 > Application config falls back to host config,
see this comment

What does falls back mean? Host config first, then app config? Just getting started. Please review the issue, I'll have question for you Thursday morning.

[serpent5]

I think it ultimately means a value will be read from host config if it doesn't exist in app config. App config overrides host config.

[Rick-Anderson]

@halter73 please review

[halter73]

I think the structure should be as follows.

## Application and Host Configuration
// summary describing config fallback behavior between config providers
// and the fallback behavior between application config and host config
### Default application configuration sources
### Default host configuration sources
### Host bootstrap settings

I also think that the section discussing default application config should be reordered to go from highest to lowest priority along with the same list for default host configuration sources. As ordered right now, the highest priority source for application config (Command-line arguments) is in the middle of the combined list of application and host sources which makes no sense. Application config falls back to host config, so I want to be able to skim down the list of sources without the order resetting in the middle.

The following should be deleted:

1. ChainedConfigurationProvider : Adds an existing IConfiguration as a source. In the default configuration case, adds the host configuration and setting it as the first source for the app configuration.

It's super confusing. Hardly anyone should care about the mechanism that application config falls back to hosting config (which isn't entirely accurate anymore anyway for WebApplicationBuilder). I think it similar to the user secrets where the actual configuration provider used to make this fallback work (if any) is unimportant.

The last (lowest-priority) element of the list of default application sources should just read:

1. A fallback to the host configuration described in the next section

There should be nothing in between the list of default application sources and default host sources other than the section heading. This should make the full list of default config sources in priority order more easily scannable.

Can we also update [App secrets](xref:security/app-secrets) to [user-secrets](xref:security/app-secrets) and Environment variables to Non-prefixed environment variables similar to in my comment?

[halter73]

This looks really good. @Tratcher said he might also take a look.

[Tratcher]