Five voices and no more

Two pulse, a triangle, some noise and a sample

The 2A03 gives a composer two pulse channels with four duty cycles, one triangle with no volume control, a noise channel and a DPCM sample channel. Everything written for the console lives inside that.

A bass line you cannot make quieter

The triangle channel has no volume envelope at all, which is why NES bass either plays or stops, and why composers used it for percussion as well.

The Noise Channel Is a Shift Register Pretending

Noise comes from a linear-feedback shift register with a short mode that produces a metallic pitch rather than a hiss — which is why NES drums sound like that.