Ukrainians are held captive in prisons, temporary holding centers, and black sites in the occupied territories of Ukraine and in russia.
With no independent oversight and basically carte blanche for how they treat POWs and civilian prisoners, russian captors feel impunity for what they do to Ukrainian prisoners, and horrific abuses result.
Russian prisons and other places of detention for Ukrainians
Prisons in Russia
Russia developed and tested this system on Russians first.
If you want to know what Russian prisons are like, visit Gulagu.net.
Here is a video from the BBC that includes research from Gulagu.net: