This is the simple schematic I've used for this project. It's quite big so you could zoom in to see the names. There are a lot of connections. Each button is connected to GND and the other side of the button to one input of the Arduino. We don't need external pullup resistors since all these inputs will be defined as PULLUP in the code. Connect power and the serial wires to the 7 segments display and finally connect 5V, GND and the LED wire to the WS2812 strip. When you make the LEDs matrix, look that the arrow on the strip will go in the correct dirrection.