Take a look at this https://learn.microsoft.com/en-us/aspnet/core/security/app-secrets?view=aspnetcore-9.0&tabs=windows

To my understanding, PaaS portal allows you to add secrets which will add or override any setting added in appsettings.json. You should be able to acess those settings using IConfiguration interface. 

There are couple of options:

1) if you want settings file to be part of the project, add specific appsettings.something.json and apply it on app startup while checking if the development environment is used.
2) if you don't want to commit as a separate file to the repository and make them visible only to you, then you can use User Secrets.

Anyhow, app secrets added from PaaS Portal should override any existing key added to appsettings.json or appsettings.environment.json.

Links:
Add app settings and connection strings
Safe storage of app secrets in development in ASP.NET Core | Microsoft Learn

Yeah, Milos, this is the right answer after working with the system yesterday afternoon. 

From reading comments in the forums, where I got confused is that .Net in general supports the idea of the IOptions pattern for injecting settings from AppSettings.json into services and the like.

While it also supports just getting them straight out of the Configuration object.

Once I got ahold of that infromation, it all made sense!