A Swiss Travel Pass is valid on all Swiss trains, buses, boats, trams and some cable cars. There's some discount for Jungfraujoch, Schilthorn, Pilatus, Gornergrat and such. Simply board any train you want and find a free seat.

A Interrail/Eurail Pass is valid on most Swiss trains + boats on Lake Thun and Brienz. The major exception is that you only get 25% discount from Interlaken to Lauterbrunnen/Grindelwald and above.

Regular tickets are valid on the route chosen for the whole day. You can stop along the way and take the next train 1h-2h later. They do not sell out and aren't more expensive on the day so no need to buy them in advance.

Supersaver tickets are cheaper than regular tickets but they are tied to a specific connection and need to be bought in advance (non-refundable). For example the 09:04 Bern - Interlaken train.

For all these tickets board the train and find a free seat.

Reservations are optional in Switzerland : nobody does them and it is a complete waste of money.

Let me know for other questions

I advise people not to book tickets in advance for example to Jungfraujoch (150 CHF and more) as it will be a complete waste of money if the weather is bad. Tickets cannot sell out anyway

EDIT Regular tickets are expensive (without half-fare card) so a Swiss Travel Pass is quickly worth it and it gives you so much flexibility. Let me know for routes recommandations too

Thanks very much for your reply. With an  Interrail/Eurail Pass specifically, I'm still not sure whether i can book tickets on the day of travel and then change or cancel them if I want to.

Thanks

You do not need to book anything. You simply board the train you want, add it to the app and find a free seat. You can stop at every stop along the way if you feel like it.

The Interrail/Eurail is basically your ticket. Btw if you live in the UK you need an Interrail pass

Brilliant, thanks. Can't wait!

You're welcome. Enjoy your trip !