Hi,
It is Ubuntu 18.04 in the default rootfs. All packages are included in SD card image. If you would like to install only required packages, please install system image through SDKManager and then install required debian packages. The packages are listed in
NVIDIA Jetson Linux Developer Guide : Over-The-Air Updates | NVIDIA Docs
Here are stpes of installing minimum packages for DeepStream SDK:
DS app running on SD, not running on eMMC - Part II - #5 by DaneLLL
FYR.