You can use every train in Italy of Trenitalia and some other partners, but notice that you need a compulsory reservation on long distance trains and high speed trains like Frecciarossa. 

And how do I make the reservations with interrail pass?

You can make the reservation on the railplanner app or at a major train stations in italy like in Milan Milano Centrale, Milano Porta Garibaldi …

You need that reservation on all Frecciarossa, Frecciargento and Frecciabianca trains (10 Euro); on all Intercity's (3 Euro) and on all IntercityNotte-Trains.

I recommend to check the prices of the private Company "Italo" (not included in the Interrail-Pass) https://www.italotreno.it/en, because there the prices are very cheap (especcialy if you buy than one/two weeks before travelling)

Note that you'll need for all Frecciarossa, Frecciargento and Frecciabianca Trains, Intercity's and IntercityNotte-Night Trains and for all Italo-Trains the digital COVID Certificate of the EU. 

On regionale Trenord and regionale Veloce we need that reservation too?

Thank you

No. On Trains of Trenord, Regionali (R) and Regionali Veloci (RV) you don't need a reservation. 

Many thanks,

It hepls a lot.