The Swiss Travel Pass is not fully covering all travels in Switzerland. It is covering public transportation to inhabited places but not beyond. It grants a rebate on mountain trips, some are included (e.g. Mt. Rigi or Mt. Stanserhorn). You can find the detailed validity map here: https://www.mystsnet.com/en/download/area-of-validity-swiss-travel-pass-2023/?wpdmdl=48956&masterkey=639077dc98f58
If you buy the Swiss Travel Pass you will not need the Swiss Half Fare Card. Choose either one of the two. Which one is better depends on your itinerary and if you are okay to buy individual tickets for each trip.
Neither the Swiss Travel Pass nor the Swiss Half Fare Card are available in the SBB Mobile app. You must buy them from the webshop on sbb.ch with your account. After purchase, once you log-in to the SBB Mobile app using the same account, the Swiss Travel Pass or the Swiss Half Fare Card will be displayed there as tickets.