英语句子中的in which和for which应该如何理解？

60天口语社群有同学提到了一个定语从句：One trend we're noticing with nearly all age groups is the growing popularity of holidays in which clients do some kind of specialised activity.句子中in which应该如何理解？这其实是定语从句中一个很常见的知识点：介词前置。上面的句子可以拆为两个分句：(1) One trend we're noticing with nearly all age groups is the growing popularity of holidays. (2) Clients do some kind of specialised activity in holidays. 这两个句子合并为一个，就变成：One trend we're noticing…the growing popularity of holidays which clients do some kind of specialised activity in. 以前的语法学家认为，介词不能放在句子最后面，因此人们通常将介词提前，句子就变成了：One trend we're noticing…holidays in which clients… 其中which指代holidays又比如有这么一个外刊句子：The aim of letting the world warm no more than 1.5°C, agreed on in Paris six years ………………………………