https://gzhls.at/blob/ldb/f/9/e/5/4607183303fb633238a8f97e0d196d0984a8.pdf

The ESP32 chip features 40 physical GPIO pads.

The IO_MUX, RTC IO_MUX and the GPIO matrix are responsible for routing signals.

This chapter describes the signal selection and connection between the digital pads (FUNC_SEL, IE, OE, WPU, WDU, etc),

256 peripheral input/output signals (control signals: SIG_IN_SEL, SIG_OUT_SEL, IE, OE, etc),

fast peripheral input/output signals (control signals: SIG_IN_SEL, SIG_OUT_SEL, IE, OE, etc),