The main difference between the two is specialty.
Orthotics are specialized products for structural or biomechanical concerns of the foot and ankle. They address flat feet, recurring injuries, and complex foot deformities, and are often used by athletes to sustain their high activity levels.
Inserts are more general and used for everyday support by people who spend a lot of hours on their feet. Inserts are not specifically made to address any medical or biomechanical issues.
Foot orthotics also provide better support, fit, and healing because they are prescribed by podiatrists and physicians taking into account your unique symptoms, diagnoses, foot shape and size, activity level, support needed, and many other factors.
Inserts, on the other hand, do not require any prescription and are easily available.