Whenever required you need to add seat reservations. Without them either you can't board the train or you get a big fine onboard. Seat reservations are mostly mandatory in Spain, France and Italy + on some cross-border routes. They're optional in Germany but often a good idea if you don't want to stand.

Have a look at those links :

https://www.seat61.com/interrail-and-eurail-reservations.htm

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

Do not hesitate for questions, mentioning travel date and route.

And these seat reservations are free or you have to pay something in addition to reserve these seats?Because if so, to the price of the global pass it would be necessary to add what you have to pay in each of the trips in which you make a previous reservation, isn’t it?

They are not free. Prices are available on those links above, from 10€ to 30€ on some routes. You need to take those prices into account but even then they have a fixed price (except in France where the 10€ fare is limited, then it's 20€) while regular tickets vary like air fares.

Book well in advance on popular routes. There are always slower reservation-free alternatives (except Eurostar). There are sometimes a quota for passholders but it is rare : Thalys, Eurostar and TGVs from/to Belgium.

Some reservations are free of charge (UK, Greece, 1st class optional reservations booked via ÖBB), but most have to be paid.