How does curry shop keep their curry hot without reducing them?
Many curry shops and other varieties of fast service restaurants and take out places will indeed keep a variety of broths, soups, stocks, and sauces hot during service hours.

Other components of the dish are kept cold, and heated when the dish is ordered. Some parts may need to be quickly cooked, then the broth added. A curry shop will do exactly this.

When you order a green curry with chicken, they will take a portion of chicken and some vegetables, sautee them until the vegetables are cooked and the chicken hot, then add the sauce. The sauce might be cold or hot, depending on how busy the place is.

Now, to really get to your question...

If a sauce or soup is kept warm for hours, you are exactly correct, it will reduce and get overly salty. There is a very easy fix!!! Add water !!! In any restaurant I have worked I would add water or stock throughout the service period. I would be tasting as needed and adjusting, but it just that simple. Add water. (or heavy cream for cream soup)