If you want to make your playbooks/roles more universal, there’s a generic package module which will figure out what package manager to use based on the detected OS.

Or, if that doesn’t fit your needs, you can add conditions to tasks (or blocks of tasks), like

when: ansible_os_family == "Debian"

and use that for tasks specific to a given Linux distro/family.

Ansible will detect a lot of info about each host and make it available as facts. See for example https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_vars_facts.html

No, you give the AIO container access to your docker daemon and it will create / handle / supervise all the other containers nextcloud needs.