I booked several domestic and international flights last year on American. American like most other airlines (I think) assigned seats automatically. After booking you can go see your assigned seats for your flight and other seats available. If necessary, then you can make necessary changes.

Booking my flight on Qantas the website assigned the two seats automatically. I then had to call the Qantas office in Phoenix and asked the representative for a better location on the aircraft.
BTW: I like Qantas, but I also like Boeing and would never fly the A380.