No. On Trains of Trenord, Regionali (R) and Regionali Veloci (RV) you don't need a reservation.
On regionale Trenord and regionale Veloce we need that reservation too?
Thank you
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.
And how do I make the reservations with interrail pass?
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.