0-inventories.monitoring.yml

---
all:
  vars:
    ansible_connection: local
  children:
    monitoring:
      children:
        dev:
          vars:
            env: dev
          hosts:
            host_1:
        prod:
          vars:
            env: prod
          hosts:
            host_2:

1-vars.env_vars.yml

dev_vars:
  hostname: monitor.dev.com
  pass: 12345
prod_vars:
  hostname: monitor.prod.com
  pass: 6789
user: monitor

2-monitoring_playbook.yml

- hosts: monitoring
  gather_facts: false

  vars_files:
      - "vars/env_vars.yml"
  vars:
    env_vars: "{{ lookup('vars', env + '_vars') }}"

  tasks:
    - name: Create user
      debug:
        msg: "I would create user {{ user }}"

    - name: Copy file to relevant environment
      debug:
        msg: "I would copy template with values {{ env_vars.hostname }} and {{ env_vars.pass }}"

3-result.txt

$ ansible-playbook -i inventories/monitoring.yml monitoring_playbook.yml 

PLAY [monitoring] **********************************************************************************************************************************************************************************************************************

TASK [Create user] *********************************************************************************************************************************************************************************************************************
ok: [host_1] => {
    "msg": "I would create user monitor"
}
ok: [host_2] => {
    "msg": "I would create user monitor"
}

TASK [Copy file to relevant environment] ***********************************************************************************************************************************************************************************************
ok: [host_1] => {
    "msg": "I would copy template with values monitor.dev.com and 12345"
}
ok: [host_2] => {
    "msg": "I would copy template with values monitor.prod.com and 6789"
}

PLAY RECAP *****************************************************************************************************************************************************************************************************************************
host_1                     : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   
host_2                     : ok=2    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

You are printing values as 'env_vars.hostname', I know this way. I am trying to use the variable as it is 'hostname', not env_vars.hostname. I am looking is there a way to load the array for the task and read the values as a string. like below.
I know this won't work, just a rough plan.

- name: print values
  debug: msg={{ hostname }}
  when: "'dev' in group_names"
  vars: {{ dev_vars }}

- name: print values
  debug: msg={{ hostname }}
  when: "'prod' in group_names"
  vars: {{ prod_vars }}

'
Let me make this simple. Forgot about environments, Let's say there is a template & variable file as below and how can we copy it, two hosts, without changing the template & without declaring sub groups in inventory.

Template:

hostname={{hostname}}
pass={{password}}

varsfile:

host1_vars
 hostname: server1 
 pass: 12345

host2_vars:
  hostname: server2
  pass: 9875

Inventory

[dev]
host1
host2

What you are describing is exactly how host_vars and group_vars work but you don't want to use them. Well then... good luck ! :)