Just to give you some idea of actual operation of such a spa...
Sun night it was 40 deg ambient temp, we had the spa heated to about 97, the rest of the pool was 60F. you could stick your arm over the inset bond beam (into the pool) and WOW was that water cold! We never felt it in the spa, it was toasty.
The key is designing the whole thing so that there is only 1/8 to 1/4 inch of water over that bond beam. and getting the valving right so that in spa mode water is only drawn from the spa and only returned to the spa. Then the heat loss of water going out or in via that beam is for all intents and purposes nil (or very close to it).
I notice in some of those pics the beam is actually raised above the level of the water and coping applied to the top. In those cases there is probably some sort of plumbing to equalize the water levels between the spa and the pool. The same applies in those cases, when the spa mode is on, the water is only taken from the spa and returned only to the spa, anything else and you will see significant water exchange between the spa and the pool (and more difficult heating of the spa because of it).