How to use Swiss Travel Pass?

For trains, you just get on the train and show your pass + ID (drivers license works) whenever someone comes through to check tickets. For cable cars/funiculars that are covered by the STP you'll need to scan your pass to get through (you'll know because it's the only way to get on). If it's one where you get a discount but doesn't fully cover, then go to the desk to have them help you purchase the correct ticket. The workers all speak English.

Generally speaking on most public transit you just board, take a seat, and show your pass to a ticket inspector when requested. You will notice that often you do not get a ticket inspection at all, and that on most busses and trams people just board through all doors, and do not show a ticket to the driver. The whole system is a basically based on "self control". You are yourself responsible for having the right ticket, and if you are caught without it the fines are hefty.

But there are no gates, turnstiles or check in totems on Swiss public transit.

For cable cars this varies a bit. These usually do have a turnstile, as they need to count the passengers. And you may have to show your pass to member of staff to be let through, or you can scan the QR code. But for most cable cars you will need to buy a ticket anyway, as most of them are not included. The STP does give you 50% off usually.

Oh yes, and what about cruise boats on Lake Lucerne?

Public boats on the Vierwaldstaettersee (Lake Luzern) are just like trains. Walk on board and when the ticket inspector comes by, show your pass, and ID if requested. Upper deck is 1st class and the main deck is 2nd class.