I am writing this guide because I struggled to find a resource that included ALL of the necessary steps to completely separate the frontend from the backend. After reading guides like Configuring different sessions for backend and frontend in yii-advanced-app and yii2 configuring different sessions for backend and frontend in yii advanced application template, there are still steps missing. It is funny how the 2nd one says it is an extension of the first one's article. So I guess, my wiki guide here is yet a 3rd and more complete extension of both of them. I also found a few StackOverflow questions and blog posts regarding this topic. None of them fully worked, and must be missing some magic sauce!
There are a few issues with the other solutions I originally used that I found from other wikis. I address the issues I had in this much simpler and shorter way. I am also going to explain what is going into way more detail than others to help people understand what's going on.
If you develop rather complex applications with Yii2, you might already be familiar with yii2-app-advanced application template. Well, the template is awesome and provides high flexibility regarding code/configuration sharing among parts of an application.
When one needs to select a record in a table that do not have related records through a relation, it is generally suggested to use a subquery.
Personnaly, I prefer a high level approach, and I have a method to do it with scopes.