Sensor Fusion

IMU attitude estimation is often summarized as: fuse the accelerometer and gyroscope.

That is true, but too vague. The real problem is: the gyroscope is fast but drifts, the accelerometer gives a long-term gravity reference but is polluted by motion, and the magnetometer can correct heading but is easily polluted by local magnetic fields.

Attitude fusion is not simple averaging. It decides which sensor to trust at which time scale and under which conditions.