where is used to describe a physical location.
Tom flew to Bermuda, where he mysteriously disappeared.
in which hrmmmmmmm that's a toughie. which refers to a clause usually
The epidemic AIDS infection rate, which is increasing yearly, has caused increased research activity in viral infections.
Oh I thought of one
Tom's muscular appeareance, in which he takes great pride, has been acquired through years of dedication and hard work.
Tom's muscular appeareance, which he takes great pride in, has been acquired through years of dedication and hard work.
Basically you can move the in to the front of the sentence or the back. so anytime you have a dependent clause that ends in 'in', and begins with which, you can move it to the front.
Not 100% sure but it makes sense to me