The few things that make up sound design include; dialogue, foley, diegetic, non-diegetic and internal diegetic/ subliminal.
Thoughts/ flashback playing in a character’s head this would be recorded in a recording studio outside of the scene then played over it.
Diegetic and non- diegetic sound
Diegetic sound is the sound that is natural produced within the film for example; singing, dialogue etc. None diegetic sound is any sound that is not natural in that world e.g the score of the film or any narration.