Using a relay module to control mains-voltage devices via a low-voltage Arduino signal, with important safety notes.
Wiring an addressable WS2812B strip with just three wires, and controlling each LED's color and brightness via the FastLED library.
An explanation of the internal structure of seven-segment displays in both types (common cathode and common anode) and how to wire them to Arduino.
An explanation of how a standalone joystick module works internally and wiring it manually to Arduino with five pins.