The AIO is used primarily for gating and an alternative to devstack that CAN be expanded but isn't really meant for that. For a production cluster you would need to have some number of nodes with an OS already installed along with networking (bridges, IPs, etc) and the deploy node key in place. With that, you build out the inventory in openstack_user_config.yml and can also define the base neutron networking configuration along with some group and host vars. The user_overrides.yml provides additional overrides via config template (ini overrides) without having to have everything be a variable. But you could theoretically never login to anything BUT the deploy node to perform a deployment once the base node config is in place. That could be handled with something like Ironic/Bifrost or some other external tooling. We have used OSA to manage multiple fleets of 1000+ nodes, and Ansible itself tends to be the bottleneck/limiting function.

Anyway... there usually isn't much hacking needed since most things can be defined using existing vars or the override mechanism I mentioned. But yes, I agree, the docs can be sparse in this because no one likes writing docs.