Question from the Docker compose test

Consider the following docker-compose.yml file:

services:
  app:
    image: my-app:latest
    environment:
      - DB_HOST=db
      - DB_PORT=5432
    ports:
      - "8080:80"
  db:
    image: postgres:latest
    ports:
      - "5432:5432"

How does the app application connect to the db database?