What is the difference between installing software using the software center and apt (or apt-get)?
In Ubuntu I added a software source to install the last version of Krita and I could get it only by using apt. In the software center I still see an old version.
So, why am I not getting the same version using apt and software center?
Ubuntu software center is a version customized by Canonical, and packaged as a Snap since several Ubuntu versions (currently 20.04 LTS and 21.10).
It is a front end to apt and apt-get, but at the same time also to snap. Thus, it can install both traditional APT packages from official Ubuntu software sources and added PPA's alike, but also can install software from Snap, the new universal software packaging system of Canonical.
Snap software is designed to run on several linux distributions, and is updated regularly. APT software remains at the version of your Ubuntu distribution, except for security updates, and except for Firefox. That is why you see a newer version along with an older in the Software center. You see the Snap version and the APT version. With apt get, you only have access to the APT version. Also there, you can have a newer version provided a PPA for that is available and enabled on your system
The difference betweek installing software using the Software Center and Apt is that they are completely different sources, completely different package types.
The Software Center is the same as using Snap (e.g. snap install [package name], as well as some apt ppa repositories. Snap packages also run on many different distributions, unlike apt repositories.
Apt can take software from any ppa repository you choose to add with sudo add-apt-repository, but by default the only repositories it downlaods from are the default Ubuntu repositories like multiverse. (Apt installs .deb packages that are in repositories)
