ATP assigns instructors to locations where they are in need and where the instructor will get the most flight time. The company does try to meet location requests but cannot guarantee it. Also, if you are working at one location and your preferred one opens up they will transfer you if you are next on the list for that location. I worked at three different ATP locations, I started far away from home and eventually got back to the location where I was from.
As a pilot you will apply to the airlines individually, you can apply to the ones that are closest to where you live or be less specific and apply to all of them. I applied to everybody and took the first one to offer me a job, but other people are more specific and wait until they get hired at their airline of choice.