The details vary, but the broad outlines are the same across the prisoners. They were taken by the Abhva (Pretya and Makara) in transit between worlds, or on sparsely settled outpost colonies. These prisoners survived being taken (many did not, and many more were consumed in spirit or flesh by the Abhva along the way). They were kept in barely livable conditions but at least they were mostly kept together. Until they were separated, and those who were strong enough to work were sent to Ice Station Zebra.

Possibly relevant details about the big picture:

• There were occasional sightings of “Administrators”, tall figures in cloaks and masks, whom the Makara deferred to.
• The Pretya seemed to ignore the Administrators for the most part, but then gain the Pretya seemed to mostly ignore the living, when they weren’t trying to devour them.
• The Makara definitely have a spoken language.
• The Pretya listen to the Makara, but don’t speak
• There weren’t any Kravyad in the group of prisoners, just Janya Artisans and Outcastes.
• Some of the Janya insist they heard the Kravyad language being spoken during transit, but not once they arrived at the mines.
• Somebody prepared the mines for the prisoners to work here, and it doesn’t seem like it was the Abhva. And it doesn’t seem like it was all that recent, either,
• There were less than a dozen Makara, more than a score of Pretya, and an Administrator controlling about twice as many Janya working a uranium mine. Once a week, an Abhva cargo vessel showed up to take the extracted uranium ore from the tunnels.
• Needless to say, most of the surviving Janya are in pretty poor health.