The Tracker One is a complete sealed unit containing the Tracker SoM, a circuit board (Tracker Carrier Board), battery, and antennas.
The Tracker SoM is the module containing the nRF52840 MCU, cellular module, Wi-Fi module, CAN interface, RTC/watchdog, etc…
Tracker Edge is the firmware that runs on the Tracker SoM. On most other devices, you write all of the firmware, starting with setup() and loop(). On the Tracker SoM, because there are so many built-in peripherals and things like event queuing, configuration service, those are all in the Tracker Edge firmware. You can write to the bare hardware if you’d like, but most of the time you’ll either use the Tracker Edge firmware unmodified or with only minor additions such as additional sensors.
Device OS 2.0.0-rc.3 should be released in the next few weeks with Tracker support. You can build from source 2.0.0-rc.2 for the tracker now, but there is no Workbench or cloud-compile support for 2.0.0-rc.2.