Jolla is a semi-open hybrid Android+GNU/Linux phone that uses among else Qt, systemd, btrfs. Its OS is called Sailfish OS, which is built on top of the fully open Mer project and its Nemo Mobile components. Jolla uses Android kernel and libhybris to use Android drivers from the GNU/Linux side.
Libraries / middleware
List of packages in the original firmware: https://people.debian.org/~timo/Jolla/jolla_packages.txt
Of those, most jolla-* and sailfish-* packages are closed, and most others are free.
For Debian, among else Nemo Mobile packages built on top of Qt 5 might be interesting to package.
A large portion of the Jolla store contents are free software, in github and elsewhere.
Devices running Jolla
Sailfish components becoming opened: https://github.com/sailfishos
Nemo Mobile middleware / UI components: https://github.com/nemomobile
The Mer project: http://merproject.org/
Full sources for the open components http://images.formeego.org/jolla/sources/
OBS for the Mer project: https://build.merproject.org/