Grammar question thank you?

"That is a sunflower 'which' is a type of plant"
"That is a cabinet 'in which' we store things"

which in the middle of a sentence is used for clarification, and sometimes as a synonym for that.

in which is a clever way of adding an additional detail to the main point.

Rearrange the sentence. If "in" can sensibly be placed at the end of the sentence, then "in which" is proper. Sticking "in" before "which" is just a way to avoid the no-prepositions-at-the-end-of-a-sentence rule. "In which" and "where" are not the same, but "in which" and "wherein" are the same. However, "wherein" is nearly obsolete -- today it's usually used only in legal language.