74HC 4051 8 channels multiplexer

Note: these multiplexers exist in different forms. The idea is always the same, but pins can be located and marked differently. Surprisingly.

What I use looks like this:
CJMCU-4051 74HC4051 8 Channel Analog Multiplexer
https://robozone.in/shop/cjmcu-4051-74hc4051-8-channel-analog-multiplexer-demultiplexer-breakout-board-for-arduino/

How the multiplexer works?

It has N (in this particular case, 3) command pins. Pins are binary, they accept 0 or 1 (LOW of HIGH signal) and therefore, we can use 3 pins to pass 2^2 = 8 different combinations, like :
0, 0, 0 (LOW, LOW, LOW) for 0
or
0, 1, 1 for 3.

The binary values are used here the same way we use it in C++ bytes, so if you are familiar with Arduino programming you should know the concept: a digital value equals 2^0 * voltage_on_first_pin + 2^1 * voltage_on_second_pin + 2^2 * voltage_on_third_pin + ...

So to encode 7, we need 2^0 + 2^1 + 2^2 (HIGH, HIGH, HIGH).

On the following picture you can see the 0, 0, 0 command turning on the 0th input (X0).

In the same time, if we provide the 1, 1, 0 command, X3 is activated.