Depending on place/time, the ATPL theory is/was done after the CPL. In Canada for example it is done after the CPL and 750 hours of flight last time I flew there.
Doing it after the PPL, like it's the rule now in Europe, leads to 2 main problems: you won't really understand what you memorize, even if you think you do get it. Second problem is that you will forget most of what you have studied when you will be in a position to use your ATPL (multi crew command).
When I see individuals wanting to start the ATPL theory even before the PPL makes me smile.
Theory should match any time the practice level, especially for the pilot training! But in Europe and some other place theory and practice are two strangers with no link, they are completely disconnected. That just doesn't make any sense.
If some person in charge is reading that, here is what I think:
PPL theory, then PPL licence.
CPL theory, then CPL licence.
While working as a first officer, ATPL theory, focused on captain tasks and issues, then ATPL licence.
Easy, logic and simple right? That's more or less what's happening in some countries.
Today in Europe the CPL/IR theory and ATPL theory are for the most part EXACTLY THE SAME!
And most people study the ATPL at a level there is no way they can understand all what they memorize.