when to add the curry powder I heard that if I put the curry powder early, the cooking heat will destroy the flavour, and that it should be put at the end, before turning off the heat

but I can't find anywhere on the internet confirmation for this statement.

is that true ? when is the best time to put the curry powder ?
None of these is categorically true. Adding it early or late will give different flavor, adding it at the very end of cooking will usually leave you with an unpleasant raw spice taste unless it is a roasted variety of curry powder.

It does make a big difference whether the powder is added into oil, water, or an emulsion.

For the most "complete" flavor, add it twice - part goes in the oil when your aromatics are mostly done, before adding bulk liquids; part goes in a few minutes before taking the dish off the heat (optionally bloomed in oil).