In Windows Unicode character can be entered by pressing and holding ALT key and typing the desired code.
In Linux to enter Unicode character press CTRL+SHIFT+U, a character like u will come. Now type your character code and you will see your Unicode character.
So to type © (Copyright symbol)
You'll see a character like u
Type the code for your character, for copyright symbol its 00A9. So type 00A9
You'll see your desired Unicode character.
In short, CTRL+SHIFT+U and 00A9