I am struggling to understand something about the advantage rule in football. What should happen when there is given advantage by the ref and after that there is a handball (talking about Ajax vs Chelsea). Should the ref give the first foul or the handball after the given advantage? What I have read so far, nothing is mentioned on that matter
The referee could going back but only after a shortest time after foul is committed. It's not an official rule. It's left to the discretion of the referee
When advantage is played, there is no "going back" to the first offence.
If a subsequent offence occurs, either that second offence is penalised as appropriate or it is again met with advantage.