That might be one of those "true but meaningless" facts.

1. The housing shortage doesn't mainly manifest as homelessness, but as housed people crowding into existing places and rural people not being able to afford to move to higher paying jobs in cities. So the number of homeless is relatively small.

2. The empty houses are in depopulated rural places where there is no reason to live.

A number I see thrown around is that California is "missing" 3 million homes right now, and people keep moving in.

Couldn't SV solve most of the homelessness virtually overnight by financing a hundred or so large homeless shelters or urban camping grounds? If every company pitched in and had it run by competent private firms I imagine they could make a huge difference, more than any taxes would do.

Shelters don't 'solve' homelessness; homes and jobs and ongoing support do.

Sure, except those things, like almost all housing construction, are illegal.