Basically, when you start an application, all the functioning parts are transferred to the RAM and is executed and run from there. This makes it easier for the processor to transfer and exchange data. However if it had used the secondary memory (internal memory or SD card) the data transfer will be slower and thus slows down the entire process.
Similarly, the system has some core functions that are needed for the device to keep running. These critical files are stored permanently as soon as your device boots up. After which, all other apps gets booted up.
You can get some part of that 1 GB memory taken by the system if you could uninstall some of the unnecessary system apps like bloatwares.
This is same across any device. Hope I cleared your question
