The main storage device inside a computer where programs and files are saved is called the

ssd or hd (you can chose) ssd and hds are bigger file sizes saving for long term data while RAM gives applications a place to store and access data on a short-term basis. It stores the information your computer is actively using so that it can be accessed quickly.

So if you buy RAM you'll get 8-32 GB of ram and if you get ssd or hd you will get around 500 GB- 2 TB (with modern storage devices)