What are those weird folders?

Question

Yii

Hello everyone, I am studying yii, in the process of studying I installed these modules:
1
2
3
now I looked into the web folder and saw this picture,

such as what is in the backend what is in the front, can you tell me if this is normal for yii ?, I just used to come across what is usually such folder names have viruses, so I think it's just modules or something else)

Answer 1

it's ok www.yiiframework.com/doc-2.0/guide-structure-asset...