How to work with multiple LED 8x8 Matrices controlled by MAX7219 and connected to an ATtiny85 microcontroller? It's not that complicated! I wrote a short tutorial about how those MAX7219 modules work,
How to control LED 8x8 dot Matrix Display with MAX7219 and Arduino | LED 8X8 Matrix Display MAX7219 - YouTube
Arduino: Led Matrix 8×8, Max7219 and Arduino Uno R3 | by Julien Louage | JLouage | Medium
Driving an 8x8 (64) LED Matrix with MAX7219 (or MAX7221) and Arduino Uno - Electronics-Lab.com
Arduino Matrix Scrolling Text Display with MAX7219 - eleobo
SOLVED]Max7219 and Power - LEDs and Multiplexing - Arduino Forum
In-Depth: Interfacing MAX7219 LED Dot Matrix Display with Arduino
How to Setup LED Matrix Displays on the Arduino - Circuit Basics
Interfacing MAX7219 LED Dot Matrix Display with Arduino
How to Interface 8×8 Dot Matrix MAX7219 with Arduino UNO
In-Depth: Interfacing MAX7219 LED Dot Matrix Display with Arduino