It's uses gn, a build system developed for Chromium. That said, on an Ubuntu system the steps to build Chromium are fairly well-documented and straightforward, as long as you have an appropriately beefy system (unfortunately it's a long wait and a fair amount of RAM is required, and Googlers tend to use a distributed build system).
https://chromium.googlesource.com/chromium/src/+/master/docs...