This is a bit of a broad question: is there something more particular or detailed you'd like to know about the Soviet housing system?

At least for starters I can say that housing was not actually free, but rather was heavily subsidized by the state (who also constructed most of the urban housing in the country) causing rent to be very affordable to effectively everyone. But the housing situation varied quite a bit from era to era, so I may asked that you narrow your question a bit.