They gonna find a new job and transfer their H1B there. It is difficult to get the first H1B as you must fit the limited annual cap. But once you got it, you can relatively easily transfer to new employer. Speaking from experience...
Not on short notice. Need to find an employer willing to sponsor and pray H1B application gets approved. Takes months. I agree it's much easier if you're in tech though.
In the context of SV, 100% of employers sponsor H1 visa. So you can literally walk to the next building, interview and have an offer by the next day. And get an approval in 15 days, and start working. I am exaggerating a little, but there are hundreds of thousands of H1 eligible software eng jobs available in SF and SV.
What are they gonna do? Quit and go back home? It's modern slavery.