Here is something about building kernel for Librem 5: Librem 5 kernel hacking
But for desktop PureOS I don’t know, I am wondering just like you, where can we find the kernel source code?
For other packages it is possible to use “apt-get source” but that does not seem to work for the kernel, it fetches something but it looks like mostly some kind of metadata, the actual kernel source code is not there.