which of the two it is?
In every Ubuntu release, there's an initial import from Debian unstable for many packages in
main. After 9 weeks or so, that process is frozen and the versions are locked down.
However, there is still tracking done by the MOTU team for universe. Many packages come across from testing. (Many packages are also wholly original too.)
For both sets of packages, bug tracking is done on Launchpad and custom patches will be introduced or backported from the previous Ubuntu release.
Neither, but it resembles Unstable more of the two.
Ubuntu syncs packages during the development cycle from unstable and those packages then get stabilized and/or modified before release. At the time of the release, Ubuntu has fallen behind of Unstable quite a bit, but it adds time to find and fix the bugs that have crept in.