Alcatraz itself was rather small, but there were dozens of other nearby camps in its area "feeding it". iirc they added the ovens later, those took a while to build.

Barbwire and military-style barracks are not hard to put up.

Even if you built dozens of "homeless cities"? Half of them would flat-out refuse free room & board for life. The ONLY solution is involuntary incarceration. Not in jails or camps but in mental institutions.