System memory on an Android device is the portion of the device’s internal storage that is reserved for the operating system and system processes. This includes the Android operating system, pre-installed apps, and other system files that are necessary for the device to function properly. It is different from RAM (random access memory), which is used for running apps and processes.
So the available memory of the newly purchased phone or the phone that has just been used is less than the declared memory value. If you do not want to use some third-party app, you can try to uninstall it to free up space.