The use of dont and duquel/de laquel/desquels/desquelles is a bit tricky. In the lesson it sounds as if you could use them interchangably but this isn't really always the case.

You need to distinguish between (1) a prepositional phrase and (2) a verbal phrase.

1. Prepositional phrases are formed by at least two words that could, in theory, be replaced by a single preposition: près de, à côté de, autour de, à propos de, etc. Prepositional phrases are replaced by duquel (and its derivatives).

2. Verbal phrases are those, in which de follows a verb: se servir de, parler de, se souvenir de, etc. Verbal phrases require dont.

In spoken French, dont often is used in places where, actually, a version of duquel ought to stand.

La fourchette dont tu te sers. -- Verbal phrase, use dont. Note that in kwiziq dont and duquel are treated as if they were synonymes. This is, according to my understanding, strictly speaking an oversimplification. La fourchette de laquelle tu te sers sounds off to me. I would like to hear Cécil's verdict on that, though. Hope she sees this.

Le garçon près duquel je me suis assis. -- Prepositional phrase, use duquel.

Dont also is used to denote a kind of implied connection or ownership of something (often times where "whose" is used in English):
Le livre dont je connais l'auteur. -- The book whose author I know.

And dont also specifies a subset of something (in English: of which):
J'ai deux chats dont un est noir et l'autre blanc. -- I have two cats, of which one is black and the other white.

Thank you so much, Chris! I remember learning this a while back, but had forgotten which phrases went with dont vs. duquel/de laquelle/desquelles/desquels. I appreciate this refresher, written in such a clear and detailed way.  Merci!

But in Chris's earlier message he said that "La fille de qui je parle" was correct. The later message says that "parler de" is a verbal phrase and therefore requires "dont".