Programming The Attiny45 With An Arduino As ISP Programmer - The EECS Blog
ATmega328P Wearable Development DIY Board + CH340 USB to TTL Programmer Module + 6 Pin Cable Compatible for Arduino|usb to|usb to ttlusb to ttl ch340g - AliExpress
Installing an Arduino Bootloader - SparkFun Learn
Arduino - Wikipedia
Direct Port Register Addressing in Arduino - Yash Vidyasagar