Reservations are completely separate from your pass. Booking or cancelling a reservation does not do anything to your pass, which you should not activate until your start date.

If you don't need a seat reservation, then there's nothing to book. Adding a train in the app does not book anything. Adding the first train of a travel day to your pass will activate a travel day; you should not do that in advance. You can cancel an active travel day (and pass activation) until 23:59 CET before validity starts, but there's no reason to activate in advance, other than to test and familiarise yourself. After that, it's best to cancel activation of the pass until your start date.

Side note: Germany to Paris is best booked on raileurope.com (desktop website only).

So if I bought a pass through Eurail.com, I can still book on raileurope.com (using the pass I bought and not having to buy something additional)?

Also when I book a seat reservation (required for train to Paris it seems), there is a fee for this. Is the fee refundable? And if I cancel the seat, then again this is not counting as a travel day?

You can book reservations wherever you like. Since Eurail add a €2 booking fee per person and train (and some reservations are more expensive to begin with) and has worse refund/exchange conditions, booking elsewhere is a good idea.

Here's an overview:

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

Reservation are generally not free of charge, with few exceptions (UK, Greece). Paris to Germany is €18.

Booking a seat does not use a travel day and cancelling a seat does not do anything to your pass either. You can book many reservations even without having a pass yet.

The ability to exchange a reservation or get a refund depends on the operator and where you book. TGV reservations can generally be exchanged or refunded before departure. Optional reservations often cannot.