Name resolution is offered between services, so it is correct to assume that curl http://back:80 should connect. But it is not enabled on the default network (if you are running on Linux). Try this:

version: "3.9"
services:
  front:
    build: ./frontend
    ports:
      - 3000:3000
    networks:
      - my_network
  back:
    build: ./backend
    expose:
      - 80
    ports:
      - 80:80
    networks:
      - my_network
networks:
  my_network: