You should not activate your pass OR any travel day until just before travelling. You never know an illness or any other unforeseen problem thwarts your travel plans.

Once a travel day has started, it’s impossible to get them back. Luckily, you can de-activate both your pass and travel days before the start of the validity period or the travel day or the pass.

A lot of people got into trouble already with this.

Anyway, answering your question:

Registering your journey in the app =/= reserving a seat on a train.

There are three categories of trains in Europe

• Trains with mandatory reservations, mostly long distance international traffic or high speed trains like Eurostar, Thalys, TGV, Frecce trains, AVE,…. You’ll need a valid ticket (=your pass) + seat reservation in order to travel on these.
• Trains with optional reservations, almost all German/Austrian/Swiss trains  (ICE, IC, Railjet, EC,...). Reservation is not necessary to travel on these, but recommended on busy times, small group or if you want specific seats.
• Trains with free seating, all the rest of the trains in Europe (most trains).

Please find all reservation details in the topic below, please read it attentively:

Paris Zürich on TGV Lyria has expensive, even extortionate, seat reservations. You can avoid these by going to Mulhouse/Bellegarde/Strasbourg (Fr) and take a regional train to Switzerland. Details in the topic mentioned above.

Btw, not entirely sure. But as you are a French resident, I think for your inbound/outbound journey you get your reservations for free. You can find them at the Guichets in the nearest station in France.