How does additive synthesis build an electronic sound?

In the context of electronic music, additive synthesis builds complex sounds by combining multiple simple signals, specifically pure tones known as sine waves that lack overtones.1

Unlike subtractive synthesis, which shapes sound by removing components from a signal that already contains a fundamental tone and its related overtones, additive synthesis constructs a sound from the ground up. By layering these individual sine-wave signals, musicians and composers can achieve precise control over the resulting timbre and produce nuanced tonal colors that are difficult to replicate with other methods. This technique is often associated with the specification of individual harmonics, a process sometimes referred to as Fourier synthesis.1