~"It's complicated".

There's really 2 different things here:

Firstly, there's "How many things are between my finger on the trigger and shooting the gun". The furthest you get is not having a round in the chamber, where you have to pull back and release the slide to chamber a round. In that state, basically nothing you do to the trigger or any other part of the gun will result in a discharge. You could use the gun to hammer a nail into a board and it would be really inefficient but also not result in you shooting anybody. From there, you can have a round in the chamber but the safety on. Some guns have no safety. Some guns have more than one (a common combination is a toggle safety and then a bar built into the trigger that must be pulled first before the trigger can be pulled). The safety's job is to stop the gun from slamming the firing pin into the back of the round. All of this matters a lot for the kind of issues that were common with Serpa holsters, where users tended to slide their finger along near the trigger and were accidentally pulling it as part of their draw from the holster.

Secondly, there's "is there anything stopping the firing pin from just smacking the round and firing it w/o the trigger or anything else being involved". In some guns, the firing pin is physically blocked from striking the round until the trigger is pulled back: there's a piece of metal or other impediment that's in the way, and when you pull the trigger it slides out of the way and then the mechanism pushes the firing pin forward. But in other guns, that isn't the case: the firing pin is held away from the round by some style of tension, but isn't physically blocked. On those guns, if you have a round in the chamber and you whack the gun in the wrong direction, the firing pin can push into the round and fire it. Sig's prior claims were that this was not possible on the P320. Evidence suggests that they are incorrect.