They use proxies who are Singaporeans who owe loanshark monies.
Eg. You find sex workers working in private apartments and condos, these units are rented by proxies. When police raid the unit, they only pass the girls to ICA.
Eg. You find syndicates bring KTV girls on social visit pass to Singapore or PRC job seekers staying in HDB/private properties run like short-term coliving or airBNBs, these units are also rented by proxies.
Eg. You find construction companies, eateries cramming their work-permit workers 20-30 pax to a unit (legal limit is 6-8) while registering the address on residential addresses without homeowners' knowledge, they also used proxies to rent the unit.
Very often, URA and MOM don't even bother to charge the proxies, so this problem never solved in the past ten years.