Locating source code of the PureOS installer

I have been playing with PureOS installation options (the ultimate goal is to install it dual boot with MacOS on a MacBook Air, but this is out of the scope of this question). What I would like to see is how this "Boot loader location: Boot Partition (/boot)" option actually works behind the scenes (in particular, how it makes the Mac's bootloader know where the /boot partition is, by "blessing"?): I thought that digging through the source code of that installer should shed some light on what it does. I therefore went to see what is under the "Source Code " link on their website, but it looks like a packages repo rather than source code: So, the question is: where is the source code of the PureOS bootloader installer? Am I just missing something obvious, or is it not actually available?