I know the Execution Order of MonoBehaviour Event Functions. So, I know that all OnMouseDown/OnMouseUp events are executed before all Update functions. However, I have script ‘A’ whose OnMouseDown event raises a custom event (delegate based) which is handled by ‘CustomEventHandler’ in script ‘B’. Will ‘CustomEventHandler’ execute before or after Update in script ‘B’?
Nicolas it is in theory 
, as for the answer the delegate or subscribed method will be called within the execution of the OnMouseDown of scriptA. It is logical since script A reaches a point triggering the event, then the event calls in all subscribed methods (if many or any) one after the other they do their things and return to leave the place for next subscribed method until they are all done and the OnMouseDown method return.