What are the built-in inventory groups all and ungrouped?

Ansible always creates them: all contains every host in the inventory, ungrouped contains every host that belongs to no other group.

You never declare them, yet you can target them:

• ansible all --list-hosts lists the whole inventory. In the lab inventory with servera to serverd that gives four hosts.
• ansible ungrouped --list-hosts shows hosts that were listed outside any [group] section. When every host has a group, the answer is the warning "No hosts matched, nothing to do" and hosts (0).

So every host is always in at least two groups: all plus either ungrouped or one or more real groups. ungrouped is a handy sanity check: a host that shows up there was probably meant to be filed under a group and was forgotten.

Go deeper:

• Default groups (official docs) — all and ungrouped, and why every host belongs to at least two groups.