How Environment variable names reflect the structure of an appsettings.json - asp.net-core-5.0

Question

I am using ASP.NET Core 5.0 and I have a Web API app deployed to internal cloud where few settings like DB are controlled via environment variables on the host cloud. In my Startup.cs I have the below code
string projectDbConnection = Configuration.GetSection("ProjectDatabaseSettings").GetValue<string>("PROJECT_DB_CONNECTION");
string projectDbName = Configuration.GetSection("ProjectDatabaseSettings").GetValue<string>("PROJECT_DB_NAME");
Here as I understand, when running locally in IIS Express it looks for appsettings.<Environment>.json and they take precedence over appsettings.json values.
But this app is always connecting to the wrong DB when I deployed to Cloud where I mentioned the PROJECT_DB_CONNECTION & PROJECT_DB_NAME as Environment variables for the app.
To make the app read from the Environment variables I had to change the above Code in Startup.cs as
string projectDbConnection = Configuration.GetValue<string>("PROJECT_DB_CONNECTION");
string projectDbName = Configuration.GetValue<string>("PROJECT_DB_NAME");
I am unable to understand the difference between the GetSection.GetValue and just GetValue and why I should use Configuration.GetValue() to direct app to read from Env variables.
what am I missing and when should we use what?

Answer 1

Naming of environment variables
There is kind of a naming convention in the environment variables for nested appsettings to env vars, see naming of environment variables.
Each element in the hierarchy is separated by a double underscore.
In your case it would work if you name the env variable: ProjectDatabaseSettings__PROJECT_DB_CONNECTION.
Config Order
Regarding to Microsoft Documentation there is a order in which the config sources are checked.
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.
appsettings.json using the JSON configuration provider.
appsettings.Environment.json using the JSON configuration provider. For example, appsettings.Production.json and appsettings.Development.json.
App secrets when the app runs in the Development environment.
Environment variables using the Environment Variables configuration provider.
Command-line arguments using the Command-line configuration provider.
The usecase
This is useful when you are developing local using appsettings.json, but run in a cluster or cloud in production where it is more convenient to use environment variables (f.e.: in kubernetes environment variables are set via config maps).