You can use the Interrail Germany pass from following Italian cities. Return journey is also included.:

• Bozen/Bolzano
• Trente
• Verona
• Bologna
• Venice

Note this is only valid for the direct ECB trains operated by DB and OBB. Also you cannot use any regional trains until you are inside Germany, nor is your pass valid for part journeys to any stations on the ECB route not in the listing.

Also note that the ECBs travelling in Italy carry a 10 euro mandatory supplement and optional but recommended reservations.

@Yorkie the 10 Euro does not apply to the German Rail Pass. They do not have to pay it, because the train is of German Railways also in Italy.

But the question is, is it allowed for people living in Italy to use the German Rail Pass in Italy to go to Germany? Normally not with One Country Passes, but German Rail Pass is an former DB Ticket sold now by Interrail. Could work, have not to.

I merely passed on a reference from an earlier post in the community. As far as I knew all passengers with a pass have to pay the supplement as it is an Italian requirement, not DB. I am sure others will confirm the correct situation on the supplement.

It is definitely on the IR site that you can travel from the named cities on the ECB to German stations with the Germany single country pass. There is a full list of all cross border cities included on the website, but AFAIK you have to only use the DB services e.g. Brussels to Cologne. Again others may clarify my understanding.

The DB-ÖBB Eurocity are also in Italy trains of DB and ÖBB, they only ask Trenord for help in operational things. But the tariff PDF for international rides of Deutsche Bahn says only for Eurail Global Pass, Eurail One Country, Interrail Global Pass and Interrail One Country Italy that you have to pay the fee. https://assets.static-bahn.de/dam/jcr:bfd93e43-1bb8-494a-b720-1f81f0b87396/Internationale%20Bef%C3%B6rderungsbedingungen%20der%20DB%20AG%20-%20Stand%2001.08.2023.pdf

At the fees for German Rail Pass they add all other fees likes partial pass for rides to other countries except ICE to Brussel and DB-ÖBB EC, but not the fee for DB-ÖBB EC. 

On top the fee is not an italian requirement, but I think it is to get money by interrail users by DB and ÖBB that are on the italian part the carrier of the train and also to not get overcrowd like on ICE to Amsterdam or IC to Denmark without having a compulsory reservation.