Ubuntu Developers (there are many groups, see the linked page) don't compile the software on their own machines, though they do before uploading it to test it.
They upload a source package to a build farm which compiles all the packages for all the different architectures. You can see a list of uploads as an example of what packages are being uploaded to the Launchpad builders, which then build it and then publish it to the Ubuntu archive, which is then mirrored around the world.
The linked wiki page shows you the list of team, you can click on each team to see who is a developer for that team.
Ubuntu Developers as a group are composed of both Canonical employees and community contributors, though the bulk of the Ubuntu archive is imported from Debian and just rebuilt to use the Ubuntu toolchain.
The version that is included in Ubuntu usually depends on the latest stable release of something available before Feature Freeze; in some cases it can be settled on ahead of time (like the kernel), or in other cases it will be whatever the latest stable release is (like Firefox), or in other cases it will be whatever version of the package was in Debian at the time.
The choice of which packages make up a specific image is made by that project lead(s) by what seeds are selected. For instance, the Kubuntu community decides for Kubuntu. During the release cycle, the actual version to be included gets decide by developers and the release team.