That's impossible to say without knowing what journeys you intend to make.

You could make a plan, check normal fares for it and then see if a pass plus reservations (where needed) would be cheaper.

Go to each Train Comoany per country, like Spain, France, Italy; enter the city to city, dates, and choose your best time to travel, check very well the connections, time between changes trains, and book it . I change my mind about buying through Eurail because there are extra charges after you want to book a reservation.

When riding with a pass-check Country info and REServations surcharges-its for a very big part all in this site.

IN countries like DE - CH -AT -CZ -NL- BE - GB - DK you can travel without any need to pay extra, and there are frequent services to all main towns-mostly every hour same timings.

Your country ESpana, with that terrible RENFE is the very worst of charging extra and making pass-use difficult. Followed by FR with its many TGV and ITaly with superfast Frecce.

Buying the interrail pass but booking on the transport app?

I want it to travel around Norway and maybe Sweden.

For Norway, Interrail only offer paper reservations, with added costs (9€ per order, on top of the booking fee of 2€ per seat) and then they must be sent via the post. Booking reservations for Norway is better done via Entur, see this page (scroll down to Norway), who will send reservations by email. You can also book reservations in Norway at a ticket office, but there are not very many.

For Sweden, it's best to book reservations via SJ. You need a pass cover number for SJ, which you can request here.

Not all trains require a reservation though, so first check if it's needed.

So you do not recommend me to buy the interrail pass?

I didn't say that. If you're going from Spain to Norway, then it's probably worth it. But it will require some planning for the trains with mandatory reservations.

I will go by plane to Norway