Here is a simple, yet useful circuit of an Arduino 4-digit 7-segment LED display unit. The module used here is a self-contained, compact common-cathode module containing four 7-segment LED numeric ...

For testing purpose only one 330 Ohm resistor (R2) is added between ground rail (0V) and the common cathode pins (3 & 8). It is better to directly connect pins 3 & 8 of the display to ground rail.

Some things only live on the diagram, like a timer or a connection to an Arduino. But some things live on both like switches, LEDs, graphs, and so on. You can connect all the little boxes together ...