Zephyr is a scalable real-time operating system (RTOS) supporting multiple hardware architectures, optimized for resource constrained devices, and built with security in mind.
The nRF Connect SDK includes a fork of the Zephyr Project. This fork is kept as close to the original repository as possible, but it might contain some additions that are specific to Nordic Semiconductor devices and applications.
The Zephyr repository, and the nRF Connect SDK fork of the Zephyr repository, contain the low-level libraries and drivers that are part of the RTOS. See the documentation of Nordic Semiconductor’s Zephyr fork for more information.
The history of the downstream nRF Connect SDK Zephyr fork is periodically rewritten to clean up intermediate work, merges of upstream, etc. See OSS repositories downstream project history for more details.