Where does APPDATA get initialized in the default docker-compose.override file

Below I will show the default docker-compose.override.yml file that was created for me when added orchestration to my ASP.NET application. This is using .NET Core 3.1. I right clicked on the project file -> Add -> Container Orchestrator Support… and the docker-compose project was added to my solution. (Obviously I chose Docker Compose in the options).

I would like to know where the variable APPDATA is being initialized so that I can understand the full path. I understand how the volumes work; I’m simply interested in knowing how and where APPDATA is initialized.

version: '3.4'

services:
  myproject:
    environment:
      - ASPNETCORE_ENVIRONMENT=Development
      - ASPNETCORE_URLS=https://+:443;http://+:80
    ports:
      - "80"
      - "443"
    volumes:
      - ${APPDATA}/Microsoft/UserSecrets:C:UsersContainerUserAppDataRoamingMicrosoftUserSecrets:ro
      - ${APPDATA}/ASP.NET/Https:C:UsersContainerUserAppDataRoamingASP.NETHttps:ro

Source: StackOverflow