ARDUINO UNO SMD Reference Design in DesignSpark PCB format
Your first Arduino project: Design and Build a Colorful Musical Toy. | Bill Jamshedji | Skillshare
Arduino simulator AND
From Arduino Prototype to Mass Production
Understanding Arduino UNO Hardware Design - Technical Articles
Arduino and Genuino brand refresh, design insights | Arduino Blog
Circuit design software that makes it easy to design, document, and debug electronics projects for the breadboard (more info in the comments). Would you find this useful? (x-post with r/ArduinoProjects) : r/arduino
Tinkercad & Arduino: How to Design and Simulate Circuits | All3DP
ARDUINO UNO WiFi REV2 — Arduino Official Store
Collaborative Design and Creative Expression with Arduino Microcontrollers (MIT IAP 2017) | Andrew Ringler