The source code is linked to on the Zorin OS website, but it is tucked away under the 'Company' > 'About' section accessible from the site's footer. Here is the link to it: https://zorinos.com/about/#source-code
For building the OS yourself, you will need to identify the packages that you want to include. DistroWatch has a list of the packages and their corresponding versions included in each release of the OS. You could use this information and the source code repositories to work out which packages and which versions are neeeded.
I have very little experience regarding the actual build process. A resource such as Linux from Scratch may be useful as a starting point. Since Zorin OS is based on Ubuntu, you may be starting from a known, pre-built version of Ubuntu and then adding/modifying packages as necessary.
In any practical scenario, you will also need to work out some way to include updates and patches from the upstream repositories into your build. The build process will also require automation, so that you can produce builds in a repeatable manner while being able to integrate changes.