A few things:

1. By using expose, you are making the container's published ports only available to linked/networked services. This is one reason why you are unable to access it locally.
2. Instead of hitting http://express:8000/ you should try to hit http://localhost:8000. The service is being published to your localhost system and is not being served by anything by default (e.g., IIS, NGINX).

3. Add a custom defined network in your compose file instead of using links. This is now the main way to network containers together:

   version: '3' # specify docker-compose version
   
   services:
     angular: # name of the first service
       build: ./ # specify the directory of the Dockerfile
       ports:
         - "4200:80" # maps port 4200 on localhost to 80 in container
       network:
         - mynetwork
       depends_on:
         - "express"
   
     express: # name of the second service
       build:  # specify the directory of the Dockerfile
         context: ./
         dockerfile: dockerfile.be
       ports:
         - "8000:8000" # maps port 8000 on localhost to 8000 in container
       networks:
         - mynetwork
   
   networks:
     mynetwork:

   2: https://docs.docker.com/compose/compose-file/#expose

In fact your traffic is as next:

1. User browser request page from angular container, then all pages will rendered to user's browser.

2. The front javascript code using angular HttpClient to fetch the data from express container.

   At that time, although docker-compose setup a customized network for you which afford auto-dns to resolve angular & express, but this dns just works among containers, not effect for host.

   But, your augular HttpClient which call http://express was happened on user's browser which not in container, so the auto-dns defintly not work for you to resolve the name express.

For you, if you just want to open browser from your docker host, you can use localhost, but if you also want the user from other machine to visit your web, you had to use the ip of your dockerhost.

Then, in angular HttpClient you need to use something like http://your_dockerhost_ip:8000 to visit express container.

If interested, you can visit this to see User-defined bridges provide automatic DNS resolution between containers.

    **sample docker-compose.yaml**
      version: '3.5'
      services:     
        angular:
          image: "angular-alpine:0.0.1"
          container_name: angular 
          tty: true
          stdin_open: true
          networks:
            app_net:
            ipv4_address: 172.16.238.05
          depends_on:
            - express

       express:
         image: "express:0.0.1"
         container_name: express
         tty: true
         stdin_open: true
         networks:
           app_net:
           ipv4_address: 172.16.238.10

      networks:
        app_net:
          driver: bridge
        ipam:
        driver: default
        config:
          - subnet: 172.16.238.0/24

>"1. Angular will run on 172.16.238.05:4200 and express will run on 172.16.238.10:some-port. 2. modify your config.ts or parameter.ts or anyfile where you configure express url into 172.16.238.10:some-port. now your angular will connect to express."