In general, a one country pass is only valid in that one country. So you'll need a global pass to get from Germany to Italy.

Ok thank you :)

You can buy a normal Ticket to Domodossola, Chiasso, Tirano, Brennero/Brenner, Tarvisio or Villa Opicina via Deutsche Bahn. From the border station you can can use the one country pass. 

Compare the price of a ticket to the border with an global pass, and choose what is cheaper. 

Please note that in Italy regional trains are cheap (a pass could cost more, than standard tickets). For your Interrail you need on all long distance trains an additional reservation like IC (3€), and EC (IT-CH), FB, FA, FR (10€) reservation fee.

The Eurocitys and Railjets via Brennero and Tarvisio (10€ fee, you can buy it on board), reservation is optional (like in Germany). 

TrenItalia also offers a 3 or 5 days pass for ONLY its regional trains, a bit like Nahverkehr/Landestickets in DE, but its really only valid on TrIt! There are in some regions a lot of local provincial other railways, not included (also not accepting passes). These are very cheap, even cheaper as just 1 day Landesticket in DE. Some provinces also offer their own passes.

Also note that due to covid-entry restrictions now also some well-used Regional Express needs REServation-not by the exact seat, but by nr.