The main goal of this portal is to guide your first steps with Debian. It explains the basics of installation and how Debian works. This portal is designed for beginners. It uses simplified language. We recommend reading installation of Debian before starting here.
http://www.debian.org/doc/manuals/debian-reference/ch02.en.html – Debian official references
Apt - The main package management suite of tools
DebianUpgrade – How to upgrade your distribution
Note: Flatpak is distro independent project that uses sandbox-like technology for application management and deployment (even building) and also not maintained by Debian project by any direct means.