How do you get a lawyer? Especially if you're already in a detention facility?

It's not my field, but I imagine word of mouth is the most likely way. You're in a facility with hundreds/thousands of other people detained on immigration charges. Many (most) of them will have lawyers already, and they'll take on new clients. Especially the lawyers who are working pro bono with immigration advocacy groups, who I'm sure make it known throughout the facility that they're there to help people who don't already have counsel. If not, family members or friends of folks who have been detained can reach out to lawyers, or to immigration advocacy groups who can find them a lawyer, on behalf of the detained person.

If the person being detained is actually a U.S. citizen? A phone call to any news organization would result in a hundred lawyers trying to get hired by that individual after the story ran. You'd have to beat them off with a stick.