How to organize applications ?

Thank you for this thread. I have installed CI4 today with the intention to migrate my sprawling php project scripts.

My project is split into 2 distinct parts.
Public Pages
Admin Area

Given the above explanation it is my intention to do the following:
1)Create a admin folder in the root and copy all app folders/files to the new admin folder

2)Use default namespace and develop all public pages in app folder

Having done this, and having changed admin/config/app.php from public $baseURL = 'http://mydomain/'; to $baseURL = 'http://mydomain/admin/'; I am running into a 404 error when trying to access http://mydomain/admin/ that states Controller or its method is not found: App\Controllers\Admin::index

I'm a little confused as I want to be able both http://mydomain/ and http://mydomain/admin/ independantly.

What am I missing?