One thing about this is that it's nothing special/custom. You can use your regular GNU/Linux knowledge and the internet. It's all standard GNU/Linux userland.
There's plenty of information and tools on the internet for doing what you're after. You don't need one true way of doing this from Librem.
I don't like complexity so I have just a bunch of simple scripts for this. But it's also possible to use buildroot, crosstool-ng, Arch Linux ARM mixed system building, where you drive the build from the target device, but compile object files remotely on x86_64 using a cross-compiler, etc., etc.