When someone says thank you, but in fact you didn't help, how should you reply?

When others say thank you, the best and most international thing is that you are welcome or indifferent. In fact, many people are grateful for such feedback. In order to know that your manners are reciprocated, others should say something when they say thank you. It doesn't matter or you're welcome. This is the basic etiquette in primary school. But there are many such dialogues in primary school, and the more people grow up, the more impolite they are.