The core electronics
The flight controller is the onboard computer. It reads the IMU (accelerometers and gyroscopes), compares your stick input to what the aircraft is actually doing, and adjusts motor output hundreds of times per second. Everything you feel in the air comes from that loop.
Electronic speed controllers (ESCs) translate the flight controller's commands into three-phase power for the brushless motors. Most small builds use a 4-in-1 ESC that stacks under the flight controller, which dramatically simplifies wiring. A power distribution board (PDB) splits battery power to the ESCs and accessories, and a BEC provides a regulated low voltage for electronics.
Motors and propellers
Brushless motor size is given as stator width and height, like 2207. KV is RPM per volt with no load: higher KV spins faster and suits smaller props or lower cell counts, lower KV suits larger props or higher cell counts. Mismatch them and you will overheat motors and ESCs.
Propellers are described by diameter and pitch, like 5x4.3, plus blade count. More pitch and more blades mean more thrust and more current draw, which means shorter flights. Never fly a chipped or cracked propeller; the vibration destroys bearings and ruins video, and a shed blade is dangerous.
Sensors
The IMU handles rotation and acceleration. A barometer estimates altitude from air pressure. A magnetometer (compass) provides heading, which is why magnetic interference from carbon fiber, power wires, or steel structures causes drift and circling. GPS gives position for hold and return-to-home. Optical flow and downward rangefinders help indoors where GPS cannot reach.
Frames and hardware
Carbon fiber is stiff, light, and standard for performance frames, but it is electrically conductive and brittle on hard impacts, so wiring must be protected from edges. Plywood, basswood, and plastic frames are cheaper, easy to cut and repair, and non-conductive, which makes them excellent for teaching and rapid iteration.
Small details cause most first-build failures: motor screws that are too long and short the windings, missing thread lock on motor bolts, cold solder joints, and a flight controller mounted rigidly to a vibrating frame. Soft mounting the flight controller and checking every joint before power-up prevents most mystery problems.