Does it contain all files in system.img partition ?..
[Isn't] it quite the same as system.img (image of the system parition) ?
- System.img is a file whose contents are written into the partition named system (/system).
- An OTA update is not necessarily restricted to updating a system partition alone. It concerns with anything of the firmware including boot, cache, bootloader, recovery, etc.
- Not every time a full blown system.img (DAT file actually) is shipped to the end users just for every OTA update. Depending upon the type of update1, there may be a full image of system partition to be copied or just only the specific files (packed into a single file) that are to be copied. The latter type of update, called the incremental update, is the norm since Android 5.0.
In a nutshell, if you're thinking that every OTA update actually downloads a full system.img or equivalent file and writes it into system partition, then that is simply not the case.
1 An OTA update can just be a mere update (like bug fixes, vulnerability fixes) or altogether a platform upgrade (like from Android Marshmallow to Nougat).