As fitted from factory, yes everything is powered via the main battery; but if you're only intention is using the aux for a "just in case jump start", you're much better off (cheaper, lighter, smaller) buying a tiny Lithium jump pack.
The Smartpass 120S allows for what CTEK calls "Start assistance", in which the unit does an automatic direct connection (suitable for 350A) between the two batteries if the main battery "is unable to start the engine" for a period of 10 seconds. This means requires a cranking voltage of <6v on the starter battery for the connection to engage.
Of course, if you have accessories that are running down the starting battery, the CTEK unit also allows the aux battery to trickle charge the starting battery, but I can't find an exact statement of amperage provided. Given that CTEK uses the phrase "trickle charge", I would imagine this is <5A, or roughly what a fridge would draw while running, meaning anything more and it won't be able to keep up. The docs do show that it will only charge the starter battery to 12.6v, which won't ever get it quite topped off.