1)-Qatar is at the North Terminal - area E, which is American terminal together with North-D, and they are connected air-side and one adjacent to the other
2)-again, can be D or E depending on the flight, but they are connected
3)-You have to clear immigration, get you bags and follow signs to connection and drop the bags there. Short walk, pretty straightforward, and employees always there to direct.
If you don't have access to lounges, terminal D has plenty of restaurants and shops to pass your time.
Enjoy