What is Ubuntu Soyuz?

Soyuz is Launchpad's "archive management infrastructure". You use it to compile packages for PPAs, but it's also used for all the regular packages that go into the Ubuntu releases and the official repositories.

Briefly, the process as described by one of Soyuz's lead maintainers is:

1. It starts with the 'upload server', an FTP daemon that receives source packages uploaded by users using dput/dupload.
2. Sources are them passed to the 'upload processor' which verifies their consistency (packaging metadata) and stores its information in the Launchpad database.
3. the publication of the source automatically creates a build request, which is dealt by the 'build dispatching' component.
4. it passes the source to a 'builder', an isolated environment for running debuild.
5. Binaries resulted from the build process come back to the upload processor and are checked before getting stored in Launchpad.

Here's a very-comprehensive flowchart of Soyuz: