Please explain how the Blech works on yomtov-how to work it, what can be done and what cannot be done, must the flame be left on all the time, can the gas be switched off? etc. How can a person warm up food/soap on Motzei Shabbat for the seder?
For a comprehensive answer you should look in a Halachic book, (Shmirat Shabbat Kehilchata will be a good start) Warming and cooking food on Yom Tov (starts this Pesach on Motzei Shabbat) is permissible. Basically you can't ignite a fire on Yom Tov but can use an existing flame (Yahrtzite candle) to light another. Similarly you are allowed to make the gas flame bigger. Turning the flame down is prohibited and only permissible for the sake of cooking if low flame is essential (not for keeping gas costs low…) The easiest is to keep the Blech from before Shabbat till the end of Yom Tov or – get a Yom Tov timer. There are ways to try to extinguish a burning flame on Yom Tov without the timer but I don't find them too effective and there are also halachacly problematic.