NB: If you want to know more about reproducible builds, please see the official project page. This page discusses PureOS reproducible builds.
Because PureOS uses things like Coreboot and Debian packages which are already reproducible, much of the images or ISOs that are built are also reproducible. There is ongoing work to make the entire ISO reproducible and reviewed by a trusted third party. This page describes how you can set up a PureOS image build for testing reproducible builds and how to determine how reproducible the ISO images are.