Hello annamei
When ordered online, you will get a PDF with a QR Code which is your ticket. You show it together with your physical passport or national ID card (physical, thus no photo copies / no images on smartphones of it) to the conductor. The e-tickets must bear the name of the person travelling, not of the buying person, should they not be identical.
You can link it with the SBB Mobile app, provided you purchased it from SBB as a registered customer. However, it is not necessary to link it with the app. If you have a copy of the PDF available on your smartphone and show it from there, that’s absolutely fine.
With the Swiss Travel Pass you can hop on and off the public transport services at will, except for mountain excursions where it, in most cases, grants reductions yet not free travel. See the validity map to find out more:
https://www.mystsnet.com/en/download/area-of-validity-swiss-travel-pass-2024/?wpdmdl=54978&refresh=6555d79c1e61e1700124572
Generally speaking, except for Glacier Express, Bernina Express and Golden Pass Express, you do not need any seat reservations or additional tickets with your Swiss Travel Pass. More on seat reservations here:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Kind regards,
Roland