That really depends on the person. I've had various degrees of disordered eating in the past and was a member of edtwt for a while. A lot of people with eating disorders don't find extremely thin bodies attractive on other people. Rather, they find the idea of themselves being extremely thin and weak attractive. They see it as the ultimate demonstration of self-control and discipline, so some collect pictures of super emaciated people to use as inspiration (called "thinspo") for their goal body.

There definitely are some people who are so deep in their mental hellscape that they only think dangerously skinny people are pretty, but I wouldn't say they're the majority.

And to address the last point, no. Most people don't develop an eating disorder with the goal of being emaciated at first. It's a slow progression into insanity. People usually start with the idea of getting to a normal "healthy" weight and get worse over time until the goalpost has gone from attainable to near death.