Short answers, pulled from the story.
The aircraft drifted into Soviet airspace because the autopilot system remained locked in HEADING mode instead of switching to INS mode. This technical oversight prevented the inertial navigation computer from capturing the correct course line after takeoff.
Major Gennadiy Osipovich piloted the Sukhoi Su-15TM Flagon-F interceptor toward the unidentified Boeing 747. He fired warning shots and subsequently launched K-8 air-to-air missiles that struck the airliner.
Soviet search teams located the wreckage of Korean Air Lines Flight 007 at a depth of approximately 1,000 meters near Moneron Island. The jet broke apart mid-air and crashed into ocean waters west of Sakhalin Island.
President Ronald Reagan condemned the incident as a Korean airline massacre during a press conference held six hours after the shootdown on the morning of the 1st of September 1983. He labeled it an act of barbarism while denouncing Soviet actions before the world community.
Boris Yeltsin later revealed memoranda showing the Soviets recovered cockpit voice recorders by the 20th of October 1983. These documents remained hidden until President Yeltsin handed them over to South Korean President Roh Tae-woo in November 1992.