I don't know how to use the / and /home later. Like where should I download stuff in ubuntu. I want to know clearly about this stuff. Can anyone

You don't use / at all. That folder contains system files. Only system administrators may have business there.

You maintain your user files in your home folder, which is a folder named after your login name under /home. For example, if your login is eternalsouls, then your home folder will be /home/eternalsouls. In a default Ubuntu install, a "Downloads" folder is set up where you can organize your downloaded files.

External USB drives are automatically mounted when you plug in on a folder under /media/<yourlogin>. You can organize the files on a plugged in volume as you see fit.

Also any other volume set up by the administrator for data storage, and where the user is granted permission to read/write, can be used and organized as the user sees fit.

In Linux, it is in principle only the administrator who is concerned with partitions. Once mounted, partitions seamlessly are integrated into a file system. The user is only concerned with the file system. In your case, /home is mounted on a HDD. That is where all your user files will go by default.