How do you configure the Generic Host to read configuration from a custom file format or remote source before the rest of the application starts?

Configuration sources are added to builder.Configuration before calling Build(), and since configuration is fully assembled before any services are constructed, you can add custom IConfigurationSource implementations (or built-in ones like Azure App Configuration or Key Vault providers) at this stage, and even use an already-partially-built configuration to conditionally add further sources (like only adding a remote source if a certain flag is present in local configuration).

var builder = Host.CreateApplicationBuilder(args);

// Add a custom or remote source before Build()
builder.Configuration.AddAzureAppConfiguration(builder.Configuration["AppConfig:ConnectionString"]);

// All services registered afterward see the fully merged configuration
builder.Services.Configure<FeatureSettings>(builder.Configuration.GetSection("Features"));