In the following examples, each modifying clause “contains a ‘gap,’ while the head noun is interpreted as the thing which fills this gap, making the sentence complete. . . . In English, this kind of “filler–gap” structure is also found in content (or wh-) questions and several other constructions, such as clefting and topicalization.” 
The underscore (_) represents the relationship, or filler-gap dependency, between the complementizer and a gap.
The head-initialized antecedent (italicized) has a relativizer (bolded) that is the syntactic head of a full clause, the embedded clause (coloured amber).
The complementizer together with the embedded clause (coloured amber) performs the role of a complement, a subordinate clause that functions as the subject or the object of particular verbs.
A Harvard University paper by Wilcox, Ethan, et al. (2018) stated that “filler–gap dependency refers to a relationship between a filler, which is a wh-complementizer such as ‘what’ or ‘who’, and a gap, which is an empty syntactic position licensed by the filler.”
SUMMARY/OBSERVATIONS: The first two sentences have a that-clause that functions as an appositive or what is known as an expletive because the that-clause does not serve a grammatical function. In the third sentence, there is a filler-gap dependency, wherein the wh-interrogative, or complementizer, introduces an embedded clause. The word what fills a gap created by the verb, i.e., to be what. And in the last sentence, the entire clause functions as a subject, i.e., She sipped what looked like orange juice.