I had already replied in a private message I saw first:

"f you're not seeing the new timetable yet in the app, then please update it.
Additionally, please don't rely on the app for timetables. It doesn't get enough updates to be reliable. It's better to use the operators' own websites or apps. For international planning, the Deutsche Bahn website or DB Navigator app is a good choice.
Děčín has a railway station. The trains between Berlin and Prague stop there. Just ignore the map: it only shows a selection of the main rail routes.”

So, does it mean that I have to do EACH ONE of my reservations on EACH company ? And in this case, can I indicate that I only want to book a reservation and not a train ticket ?

Thank you very much for your kindness, it is my very first time with Interrail

I look at timetables and reservations separately. My first comment was about timetables.

As to reservations, I try to get them without any extra costs, which means the Interrail website and app are one of the last choices. The best way to book them varies and depends on the trains. And if they're not obligatory, I only book a reservation if it's going to be very busy (like on Friday or Sunday afternoon/evening).

For journeys between Germany and Czechia, the DB website or app are good for reservations because you can book one journey for 4 EUR, including all trains in the journey. Fill in your details and click on the "Seat only” link. Booking via Interrail would cost 8 EUR per train but DB does it for 4 EUR per journey.

For a TGV or ICE from France to Germany, the best for you would probably be to buy it at an SNCF ticket office, if there’s one close to you.