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First of all, it took a lot of effort to really understand the message you are trying to pass, I am glad i was able to, and I understand English might not be your first language, so all is fine.
To add to what every one else have said, there is nothing special about stable coins aside the fact that they serve as a means through which traders can hedge their funds against high volatility, most especially when crypto prices are going down and there's a lot of fear, uncertainties and doubt, USDT, USDC, BUSD and the rest of them are all backed by the united stated dollars, which means each USDT or USDC is supposed to be equal to 1 USD at all time, and this is also to say that, when inflation hits fiat USD, it also affects crypto stable coins since its backed 1 for 1..
Take for example, there's this candy you normally buy at a price of 1 cent each which means 1 fiat USD normally buys you 100 pieces of that candy, something then happened, and inflation hit the USD and as a result, the price of that candy goes from 1 cent to 3 cents, this simply means that your 1 fiat USD can no longer buy you 100 pieces of that candy but only 33 pieces...if during this time you hold some stable coins and you find an online shop selling that candy and accepts stable coins as payment, if you decide to patronize this online shop, don't be surprised that your 1 USDT stable coin will still afford you only 33 of that candy, same amount your fiat USD can afford you offline.