How do you set up environment-specific configurations?

Create one settings file per environment, such as appsettings.Development.json and appsettings.Production.json, and set the environment name on each machine. At startup the app loads appsettings.json first, then the file for the current environment, whose keys override the base file.

The environment name comes from the ASPNETCORE_ENVIRONMENT or DOTNET_ENVIRONMENT variable. When neither is set, the app runs as Production. Locally, the launch profile in launchSettings.json usually sets it to Development.

Code can also branch on the environment:

if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Error");
    app.UseHsts();
}

Keep secrets out of these files. Production secrets belong in a controlled store such as Azure Key Vault.