From https://frenchcrazy.com/2013/12/french-relative-pronouns-dont-qui-que-lequel-ou.html/ :

Qui

Qui can refer to people or things and is used as the subject of a dependent clause. Because qui becomes the subject of the sentence it is always followed by a conjugated verb and must agree in number with the antecedent it is referring to.

« On critique les films qui montrent trop de nudité. » →
"People criticize films that display too much nudity."

Lequel/Laquelle

The relative pronoun lequel (and all of its forms) replace a preposition (other than de) and a thing/object in a relative clause. Make sure that the pronoun agrees in both number and gender to whatever it is describing.

« Une langue est un prisme à travers lequel ses usagers sont condamnés à voir le monde. » →
"A language is a prism across which its users are condemned to see the world."

I have a partial answer that is not academic but helped me alot in learning to use lequel as a english speaker.

Effectively, just use lequel whenever you want to replace things like:

• dans ( j’ai une boîte dans laquelle je mets tous mes trucs)
• avec ( elle est la fille avec laquelle j’ai parlé hier)
• sur ( c’est la table sur laquelle il y a un gateâu)
• à traves (c’est la vitrine à traves de laquelle on peut voir le truc)
  i can’t think of anymore prepositions but ya you get the idea…

Whereas for qui, think of VERBS

• le mec qui a largué sa copine ( qui + larguer)
• le prof qui est nul (qui + être)
• le so on so forth

hope this helps. I am not native speaker so hahah might not be 100percent correct