ansible update vs combine

output.log

PLAY [localhost] ************************************************************************************************************************************************************

TASK [debug] ****************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": ""
}

TASK [debug] ****************************************************************************************************************************************************************
ok: [localhost] => {
    "msg": {
        "Service": {
            "Environment": "foo"
        }
    }
}

PLAY RECAP ******************************************************************************************************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0

test.yml

- hosts: localhost
  connection: local
  gather_facts: no
  vars:
    test1:
      enabled: yes
      state: started
      config_overrides: override1.update(override2)
    override1:
      Service:
        Environment: "foo"
    override2: {}
  tasks:
    - debug:
        msg: "{{ override1.update(override2) }}"
    - debug:
        msg: "{{ override1 | combine(override2) }}"