There’s a really good guide to the pass here: https://www.seat61.com/how-to-use-a-eurail-pass.htm

And a guide to where/ when you need to make reservations here: https://www.seat61.com/interrail-and-eurail-reservations.htm

On routes where reservation isn’t compulsory, you can often make an optional reservation. I would do this if I expect to be travelling at a busy time, or if I was in a big group. Other times, you just board and find an empty seat.

“To use the trains that don't need to book a seat” . . . no, you don’t need to make a reservation by definition.

There are 2 main types of trips to consider -

1. where you don’t need to make a reservation - these can’t sell out, you will be able to get on board, you may have to stand and be stuck between 2 smelly people for 5 hours . . . but you can get on the train and you will get to your destination.
2. where you must make a reservation - these can sell out, you may be able to get on board without a reservation but you will be hit with a fine when the inspector finds you don’t have a reservation.

Think of #1 as like getting a local bus . . . you will get on unless there is literally no standing room and #2 as an intercity bus where the driver won’t let you on if there is no seat for you.

Muito obrigado pela ajuda de vocês, facilitou bastante o meu entendimento sobre o assunto. Já consegui comprar meu passe, adicionar meus destinos a uma “viagem” e depois associar ao meu passe, já está até aparecendo os dias com as passagens e até consigo abrir o qrcode do trem, mas tenho algumas dúvidas:

1- Fica aparecendo tipo um relógio em cima do qrcode, com uma contagem crescente, isso seria o quê? devo me preocupar com alguma validade ou algo do tipo?

2- Fiz reservas de assentos em trens que eram necessários, logo tem um custo adicional além do passe, como faço para pagar essas taxas da reserva de assento? Tenho que acrescentar crédito no meu passe e chegar na hora de usar normalmente, é isso? (Se sim, como faço pra colocar o crédito?) Porque já aparece pra mim o qrcode.

3- Também faço reserva de trem sem precisar de reserva de assento, no caso eu só preciso mostrar o qrcode na hora pro inspetor e tá tudo certo né? Porque não aparece número de assento, nada..

4- No caso, vai ter um dia que eu vou fazer um bate volta em Londres, já adicionei a viagem e cadastrei no meu passe, já ativei as passagens (não fiz o pagamento ainda porque não sei como faz), apesar de aparecer lá os destinos e horários de ida e volta, não aparecem dois qrcodes. Aparece apenas um qrcode pra mim, dessas duas passagens (ida e volta), é assim mesmo? Esse qrcode eu posso usar tanto na ida como na volta?

1. The clock on the QR-code is to prevent fare dodging by screenshots. Best not activate a travel day until just before boarding your first train of the day! You never know what happens (you getting ill, severe delays, bad weather, strikes,… you risk losing travel days otherwise. They cannot be de-activated on the travel day itself, only before.
    
2. Seat reservations cannot be booked via the app. Use the Eurail reservation module or www.raileurope.com (desktop version only, see below, Portuguese version available, very recommended). Seat reservations are seperate from your pass. Show the pdf with the reservation and the qr-code of the pass upon inspection.
    
3. Indeed, just find a free seat and enjoy the journey.

4. When are you travelling to London? Get your seat reservations asap via www.raileurope.com (desktop version only, add the rail pass you have in the search module below. “adicionar pase de trem”. Raileurope also has a Portuguese version). Get them as soon as possible, Eurostar tickets London-mainland Europe can sell out weeks in advance in busy periods. Calmest days with most availability are Tuesday, Wednesday, Thursday.
   
   A lot of seat reservations can be bought on raileurope as well (Eurostar, UK, Italian trains and a lot of trains with mandatory reservation elsewhere in Europe … Good website with no additional costs and good global credit card accaptability.

No Spanish reservations unfortunately. That would be a real improvement. Also no optional reservations (Austria, Germany etc) but there are other websites for those.