I recently started learning the yii2 framework for my internship in the advanced template. But I don't really get when you need to use the backend folder instead of the frontend folder. If someone could explain it, it would be appreciated.
It's very easy.
The backend is your admin page, and the frontend is your user page.
Let me explain.
You have models, what your backend uses, and frontend use. If both have to be used, you have to use a common folder to share models with the back and front end.
When you make an app, in the front end you can create everything, that the user needs. Like registration, login, about us, etc.
In the backend, this will be your admin page. Like user search, modify rows, etc.
If any of your models need to be a user, and to admin, like a user table, you have to use a common folder. Then frontend and backend will see this model. Be careful with rules.
I hope it helped. :)