At the hot spa, you may rent a private small open bungalow with dressing room and empty pool tub. You fill the tub with the spa water. The bungalow is quite secluded from the neighbours as it's surrounded with tall shrub greenery, i.e. can you spot a thatched roof in one of the plank photos?
Refreshments may be ordered and brought to your bungalow. You may also order eggs boiled in the hotter spa waters.
To reach your bungalow you'll walk along a plank, and pass by a pond of (too hot for humans) springs, as well as a large swimming pool for soaking (the non-private common-use pool, I'd assume).
Maybe just a co-incidence but that hot soak on a hot summer day combined with heavy road travelling in general got the better of me, ending up feverish later on and needing to sleep it off nearly all day the next day, despite non-alcohol drinking!