Docker / Postgres - Trying to run 2 databases and 2 apis, cannot connect - postgresql

Question

I am trying to use my docker-compose file to run 2 instances of both my database, and my rest api, so that I can run tests on a test instance of the database.
version: "3.8"
services:
db:
image: postgres:13.2-alpine
container_name: "db-prod"
ports:
- "5432:5432"
environment:
- POSTGRES_DB=postgres
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=password
networks:
- fullstack
volumes:
- database_postgres:/var/lib/postgresql/data
db_test:
image: postgres:13.2-alpine
container_name: "db-test"
ports:
- "5433:5432"
environment:
- POSTGRES_DB=postgres
- POSTGRES_USER=postgres
- POSTGRES_PASSWORD=password
networks:
- fullstack-test
volumes:
- database_postgres_test:/var/lib/postgresql/data
api:
build: .
container_name: "rest-api"
environment:
DB_USERNAME: "postgres"
DB_PASSWORD: "password"
DB_HOST: "db-prod"
DB_TABLE: "postgres"
DB_DB: "postgres"
DB_PORT: "5432"
ports:
- "8080:8080"
depends_on:
- db
networks:
- fullstack
api_test:
build: .
container_name: "rest-api-test"
environment:
DB_USERNAME: "postgres"
DB_PASSWORD: "password"
DB_HOST: "db-test"
DB_TABLE: "postgres"
DB_DB: "postgres"
DB_PORT: "5433"
ports:
- "8081:8080"
depends_on:
- db_test
networks:
- fullstack-test
volumes:
database_postgres:
database_postgres_test:
networks:
fullstack:
driver: bridge
fullstack-test:
driver: bridge
When i run this, my prod database starts, and my regular API connects to it fine.
My test DB also starts, and I can connect to it using
psql -U postgres -h localhost -p 5433
however my test rest API wi
dial tcp 192.168.112.2:5433: connect: connection refused
The goal is to set up my go tests to run on the test DB and just clear after each test as needed, and not affect the prod db.
I am not sure if I am going about this the right way - perhaps there is a better construct for this - and if so please correct me. But regardless, I do not understand why im getting this error?
I dont get why one connection works well and the other fails?
Edit: Also interesting, i just noticed if i change the api_test container to use:
DB_HOST: "host.docker.internal"
it works. But i still dont understand why one can use a container name and the other cannot? And i cant leave it this way as it needs to work on a mac as well, and host.docker.internal doesnt work on my mac (hence why the first one was changed to the container name)