This package is a wrapper around daisy. It pulls in config information from the build.config file that is generated by the cos_customizer which is indirectly populated by the user provided cloudbuild.yaml config. The preloader takes this information and generates a daisy config using this template here:
src/data/build_image.wf.json
Once daisy is called and all resources are created, cloud-init will execute the provisioner which is the next step of the process. Once cloud-init is done and the provisioner runs to completion, control will be handed back to daisy which will clean up any unecessary resources and exit the build.
The provisioner is packaged in an image called cidata built in this directory's BUILD.bazel file. The image is created, artifacts are copied into the image, and the image is then embedded in preload.go where it is then later uploaded to GCS to create a disk to be mounted to the preload VM.
A scratch image is also built in the same BUILD.bazel file and is used for the install-gpu step so that the toolchain isn't installed onto the boot disk and is instead bindmounted and executed from temporary storage. Much like cidata, the scratch image is also embedded in the preload.go file and uploaded to GCS to allow daisy to create a temporary ext4 disk for gpu installation.