I recently moved from Debian to Ubuntu, I usually used apt to install package (and don't want to use the software center as apt it let me know which package are installed as dependencies).
But I already found something I wasn't expecting:
some package like atom and discord are available in the software center but not in apt, isn't the software center a graphic interface for apt? does it use something else?
You have just discovered "snap" packages, they are deployed by "snappy" package management system built by Canonical Ltd. (the Ubuntu company).
Snaps are containerised software packages that are simple to create and install. They auto-update and are safe to run.
Read more here: https://docs.snapcraft.io/getting-started/3876
The Software application also contains snaps, in fact
Some applications are available in both formats. In such a case in Ubuntu Software the snap will be listed first. (source)
isn't the software center a graphic interface for apt
No. Ubuntu software center also supports snap installation. See the snapcraft store. Discord on command line would be
sudo snap install discord
and atom would be
sudo snap install --classic atom
dpkg and it's understandable if some user should wonder whether the snap command is unnecessary. +1 regardless
aptitudeorsynapticmaybe more familiar to you.