Along with the external assets that you import from other programs such as image, audio files, 3D models, etc., Unity also offers the creation of internal assets. These assets are formed within Unity itself, and as such, do not need any external program to create or modify.
Let's see some examples of internal assets:
To create an internal asset, go to the Assets folder and right-click on that folder and select Create.
In this example, we will create a Triangle and a Square.
To create a Triangle asset, right click on the asset -> Create -> Sprites -> Triangle
Repeat the same process for Square, and now you should have two new graphic assets.
As we move along, we will explore more of these internal assets, since they are very crucial to build a proper game.