How to merge variables in Ansible using the `community.general.merge_variables`.

merge-vars.ansible.yml

- name: Example merge playbook
  hosts: "localhost"

  vars:
    zabbix_agent__default__host_groups: # defined as the initial value
    # will be included in the merge
      - Linux
    zabbix_agent__env__host_groups: # merged
      - Bar
    zabbix_agent__host__host_groups: # merged
      - Foo
    not_merged__host__host_groups: # not merged
      - BAM
    zabbix_agent__host__not_merged: # not merged
      - BAM
    zabbix_agent_merge_regex: '^zabbix_agent__.+__host_groups$'

  tasks:
    - name: Set host groups fact
      # the `| unique` is probably important in most use cases!
      set_fact:
        combined_host_groups: "{{ lookup('community.general.merge_variables', zabbix_agent_merge_regex, initial_value=zabbix_agent__default__host_groups) | unique }}"
    - name: Debug merged result
      debug:
        var: combined_host_groups
    
    # Output:
    # ok: [localhost] => 
    #   combined_host_groups:
    #   - Linux
    #   - Bar
    #   - Foo
    
    # Tip!
    # If you include `zabbix_agent__ansible__host_groups: "{{ group_names | list }}"` in `vars`, you will also get the hosts group memberships.