I'm going to be in Switzerland for around 10 days and I do not feel like planning my whole trip out beforehand, do I HAVE to pre-order my tickets if I have the swiss travel pass?
I live in Switzerland for 26 Jears now and have newer reserved a seat on the trains here.
To first answer your question: You don't have to reserve seats on trains in Switzerland, but you can, if you want, for an additional fee. Under normal circumstances, there is more than enough space on Swiss trains.
If you don't want to stand tough, i would either recommend to reserve trains (only for IC, EC and ICE trains) if you're traveling between 7 AM - 9 AM or 5 PM - 7 PM on a weekday or just be at the platform 2-3 minutes before the train arrives, so you get a seat.
Reservations for the local trains (S-Bahn) are btw. not possible.
In Switzerland, just like in Germany and Austria, there is no need to make reservations. If the train is full, you might end up standing, but you will never be refused entry to the train if you have a valid ticket. Reservations are always recommended, of course, and are sold separately. On the train, reserved seats (if the train has them in the first place) are shown on or above the seats. Simply pick a different seat then.
So you can simply hop on any train you like.
(Exceptions might be trains like special panoramic trains that you should definitely make a reservation for)