(covers information from several alternate timelines)
Among the technology created by the Ancient Ones was a bioweapon called the Abronath, a massive fleet of Swarm ships (as well as drones to pilot them), and equipment capable of rejuvenating the user through energy transference, forcibly draining the life from another person and assimilating some of the victim's DNA in the process.
When the USS Franklin crashed on Altamid in 2164, the three survivors – Balthazar Edison, Anderson Le, and Jessica Wolff – found the Ancient Ones' cache of technology and began using it for their own purposes, raiding passing ships to capture and drain their crews while searching for the missing half of the Abronath. (Star Trek Beyond)
Based on the design of the structures on the planet and the appearance of the Swarm drone, it is likely the Ancient Ones were humanoids. Presumably they were also responsible for the subterranean development of the planet.