Mech: On the Toto models, the entire seat assembly with its built-in bidet sits above the rim of the bowl, so it would normally have an air gap of at least 1/4" or more - - no siphon problem.
During operation, a "wand" extends (telescopes) out and downward into the bowl area, to spray upward onto your nether regions. Every time the wand retracts into the seat, it is sprayed clean with ionized water and bombarded with UV light. I do not know if there is an additional siphon break inside the seat mechanism.
Regarding hot water: the Toto models are insta-hot. Only a cold water supply line is needed, as the water is heated inside the seat mechanism. On the moderate priced versions, it has a small built-in tank where the water is kept warm. On the high-end models it is a tankless insta-hot, about 1200 watts or so, which is why you want a dedicated 15 amp circuit for the toilet.