I think I got it, but... what exactly defines a clause?
You have a clause when you have a complete thought. Usually, a clause is defined by one (no more) verb with all its actants, i. e. noun-like words that go with the verb, either nouns or adjectives. The role of every noun-like word within the clause is defined by its case and its preposition. What that role means depends on the verb to use. For example, every transitive verb accepts two roles: the subject (in the nominative case) and the object (in the accusative case) of the action. For the verb «кушать», that's the one who eats and what he eats, respectively. For the verb «целовать», that's the one who kisses and the one who is kissed.
E. g. the clause «которая работает там» has one verb, one adverb (a word that is attached to the verb and “modifies” its meaning — see “ad” + “verb”) and one nominal word: «которая». That word is a pronoun, i. e. it has no meaning of its own and stands for whomever you think of, «девушка» in this case; grammatically, this word is quite like an adjective. Since the thing that the nominal word stands for is the subject of the action that the verb means (“to work”), the pronoun is in the nominative case. Just the same, if the first clause («Он говорит о девушке», which has one verb and two nominals) was not there, in the clause «Девушка работает там» the noun «девушка» would be in the nominative case, which I believe is a very natural thing to see.
There are huge theory books (which I didn't read) about the roles of the verbal actants; above is what is necessary to understand the usage of «который». That's also useful to understand the Russian punctuation. It's pretty mechanical, a foreigner can easily learn it better than most natives.