The bootloader is a modified version of Optiboot (https://github.com/Optiboot/optiboot).

Add the following URL to the Arduino Board Manager (File > Preferences > Additional Boards Manager URLs)

https://raw.githubusercontent.com/0x6470/ATmega328PB-Arduino-Nano-Clone-with-Bootloader/master/package_m328pb_index.json

Then search in the Board Manager for ATmega328PB and click install.

If this fails, you might download the zip file and unzip it to:

C:\Users\<Your Username>\AppData\Local\Arduino\packages\ATmega328PB-with-Bootloader\hardware

I recommend following this instruction: https://sysexit.wordpress.com/2013/02/07/burning-a-bootloader-to-an-arduino-nano-using-another-arduino/

Simply select "ATmega328PB" as board, instead of "Arduino Nano w/Atmega 328 again"