Why am I able to access my Docker-Compose container via localhost in the host web browser but not in the container?

I have two containers: one for Cypress and another for my web app. I have them both set up in a docker-compose.yml file like so:

version: '3.2'
services:
  pa-portal:
    image: web_app_image
    container_name: pa_portal
    volumes:
      - productDB:/web_app/db
    ports:
      - "8080:8080"
  cypress:
    image: "cypress/included:4.4.0"
    depends_on:
      - pa-portal
    environment:
      - CYPRESS_baseUrl=http://pa-portal:8080
    working_dir: /cypress-testing

    volumes:
      - ./:/cypress-testing

volumes:
  productDB:

From the Cypress (testing framework) container I am able to access the web app using http://pa-portal:8080 but from a browser on my host the only way I can access the web app that has been launched by the pa_portal container is using localhost:8080.

Why are there different urls depending on where I am accessing from?

Is there some fundamental knowledge i need to do some research on