Optiboot (an older version) is installed by default on the Arduino Uno and (as of 2018) official Arduino Nano boards. It can be installed on all older mega8, 168 or 328 based Arduinos. As of 2019, ...

Bootloader Warning: This board uses Adafruit_nRF52_Arduino by default, with Adafruit_nRF52_Bootloader pre-burned at the factory. Using nRF5-SDK to program the board will result in losing the original ...