If you're trying to book a seat on a train in Great Britain, these are free of charge so please ignore any website that wants you pay for them. You can book on the LNER website (only LNER trains, with seat map) or on the GWR website (almost all bookable GB day trains). To be seated together, book your seats reservations in one booking.

If that doesn't work for you or if you're trying to book another train, then please mention route, date and departure time of the train(s) you're tying to book.

Hi. I have purchased an interrail global pass and am planning my trip to Europe. The route I have in mind includes many stops between Austria and Germany and I have read on many sites that in these countries very often a seat reservation is not necessary. I wanted to ask in such cases what you have to do to 'get' a ticket on a route without a reservation. Is it necessary to select that specific route as a travel day? I look forward to your help. Thank you

You always need to register any train you take in the app; that will generate a QR code, which is proof of your ticket.

Please check the basics here (supposing you have a mobile pass):

https://www.interrail.eu/en/interrail-passes/interrail-mobile-pass/getting-started

OK thank you for your answer. Do you recommend that I register all the trains I have scheduled now? Or is it better to wait? Considering that I'll be travelling between Austria and Germany, almost all the trains don't require seat reservations, but I'm afraid I might be stranded...

There's no need to do that in advance. Just do it before you board.

You can check how busy trains will be on bahn.com (Germany) or fahrplan.oebb.at (Austria).