Info from SEPE for different cases. First one when already on sick leave when employer arranges for an ERTE and the second one for situations where there is an ERTE in place and an employee falls ill.
Translation of 2nd example:
Example: Worker affected by a contract suspension between 03/01/2012 and 8/31/2012, is recognized a benefit of 720 days duration. Subsequently, he is in IT status (medical ‘baja’) from 04/01/2012 to 04/30/2012. The SEPE, delegated by the INSS, would pay the IT benefit in the same amount as the unemployment benefit. The 30 days in IT are used up from the duration of the benefit (720 recognized days of unemployment benefit – 30 days received by IT = 690 days of benefit remain as of 04/30/2012).
For the special COVID ERTEs there is a difference in that the days you receive unemployment benefits are not taken from your entitlement.