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Apart from debian stable, testing and unstable branch contains more recent version of software. Ubuntu takes unstable one and maintains its own release system. But my point is for example KDE. KDE 19 is present in Kubuntu latest, while debian unstable is still stuck at 17. So if it's based on Debian unstable, why is Debian so many versions behind?
Is the Ubuntu packaging system different? Also can you suggest any 'rolling' distro based on Ubuntu (like debian unstable is rolling)?
KDE Plasma 5.19 is currently available in Debian experimental. Ubuntu is based on Debian, but it isn't a 1:1 copy, there are differences and Ubuntu package maintainers can decide to package a newer (or older) version in Ubuntu. If you want a Ubuntu-based rolling release distro, you can use rolling rhino tool.
Ubuntu takes unstable one and maintains its own release system.
That is really a gross oversimplification. Each Ubuntu release is based first and foremost on the previous Ubuntu release. Debian and Ubuntu have their own developers with their own priorities, many developers do stuff on both sides of the fence but by no means all.
For part of the Ubuntu release cycle there is an automated process which imports packages from Debian Unstable into Ubuntu where there are no Ubuntu-specific modifications. There is also a semi-automatic system for merging changes from Debian into Ubuntu packages that do have Ubuntu-specific modifications.
However there is no automated flow in the other direction. Updates that originate in Ubuntu will not make it to Debian unless someone explicitly uploads them there.
Is the Ubuntu packaging system different?
The technical aspects are very similar. The social/political aspects are very different. Debian is very maintainer-centric (though less so than it used to be) while Ubuntu doesn't have the concept of individual maintainers for packages at all.
In this case though I suspect the issue was simply lack of manpower, I have been hearing complaints about the KDE team in Debian being under-manned for a long time.
Also can you suggest any 'rolling' distro based on Ubuntu (like debian unstable is rolling)?
Neither Debian or Ubuntu really have rolling releases as such. They have development releases of course, but what happens in those development releases is cyclic, there are periods of regular development followed by progressively stricter freezes to prepare for stable releases, followed by catch-up after the stable release happens.
The Ubuntu equivalents of "unstable" and "testing" are essentially the "proposed" and "release" pockets of the current development release.
A big difference between Debian and Ubuntu is that the cycle is far more compressed in Ubuntu. Debian spends as long on the pre-release freeze process as Ubuntu spends on the whole release cycle.
The Debian distro tries to maintain stability and security in the utmost fashion. If you need more current software you can install it from the backports repository.
https://wiki.debian.org/Backports
Though reading the Debian documentation they really shy away from installing the latest and greatest other than security patches.
https://www.debian.org/doc/manuals/debian-reference/ch02.en.html
As Ubuntu This is a good read from https://help.ubuntu.com/lts/installation-guide/s390x/ch01s02.html 1.2.1. Ubuntu and Debian Ubuntu and Debian are distinct but parallel and closely linked systems. The Ubuntu project seeks to complement the Debian project in the following areas:
1.2.1.1. Package selection Ubuntu does not provide security updates and professional support for every package available in the open source world, but selects a complete set of packages making up a solid and comprehensive system and provides support for that set of packages.
For users that want access to every known package, Ubuntu provides a "universe" component (set of packages) where users of Ubuntu systems install the latest version of any package that is not in the supported set. Most of the packages in Ubuntu universe are also in Debian, although there are other sources for universe too. See the Ubuntu Components page for more detail on the structure of the Ubuntu web distribution.
1.2.1.2. Releases Ubuntu makes a release every six months, and supports those releases for 18 months with daily security fixes and patches to critical bugs.
As Ubuntu prepares for release, we “freeze” a snapshot of Debian's development archive (“sid”). We start from “sid” in order to give ourselves the freedom to make our own decisions with regard to release management, independent of Debian's release-in-preparation. This is necessary because our release criteria are very different from Debian's.
As a simple example, a package might be excluded from Debian “testing” due to a build failure on any of the 11 architectures supported by Debian “sarge”, but it is still suitable for Ubuntu if it builds and works on only three of them. A package will also be prevented from entering Debian “testing” if it has release-critical bugs according to Debian criteria, but a bug which is release-critical for Debian may not be as important for Ubuntu.
As a community, we choose places to diverge from Debian in ways that minimize the difference between Debian and Ubuntu. For example, we usually choose to update to the very latest version of Gnome rather than the older version in Debian, and we might do the same for key other pieces of infrastructure such as X or GCC. Those decisions are listed as Feature Goals for that release, and we work as a community to make sure that they are in place before the release happens.
