Best to read up on the pass in general.Ā
https://www.seat61.com/how-to-use-an-interrail-pass.htm
You will have to pay reservation fees on the Eurostar to Paris, as well as any TGVs in France or internationally. And the fast trains in Italy have a reservation fee too.
A pass can still be good value, but you should compare the totalĀ cost, including reservation fees, to just buying tickets instead. Especially if these are your only journeys. You can check ticket prices on Trainline or Rail Europe.
By reservation you mean the seat?
Yes : sometimes they're required (high-speed trains for example), sometimes they're optional and sometimes they don't exist. It depends. :)
Have a look at this too : https://www.seat61.com/interrail-and-eurail-reservations.htm
Between and in countries yes. Have a look at the 2 links we've shared.
Within cities the pass is not valid on subways, trams or buses. It is sometimes valid on suburban trains (London Overground, Elizabeth Line, German S-Bahn) but not always -> Paris RER network is not included.
Anyway tickets within cities are cheap and it wouldn't be good value to use a pass day.
If you've got time you could plan a stunning journey across the Alps, for example via the Bernina railway: https://www.seat61.com/trains-and-routes/bernina-express.htm