1. “What's that you say?“ (idiomatic expression to mean ‘Have I heard you correctly?’)
2. “What’s that you’re holding?” (used for referring to something that the person you are talking to is holding or wearing, excerpted from Macmillan dictionary)
3. “What's that you were saying earlier?” (excerpt from our local material)
If three sentences above have the same pattern in common, I guess the that above is functioning not as a demonstrative pronoun leading a demonstrative-cleft construction, but as a preceding pronoun restricted by a relative ‘which’ clause (with the objective ‘which’ omitted). If so, I wonder how the demonstrative pronoun ‘that’ could be restricted by a relative ‘which’. Is this case similar to the exceptional usage of ‘those who’ in “Those who saw the performance thought it memorable.”?
I would appreciate it if you kindly explain the function of three ‘that’ above, especially the ‘that’ in 3.
Additionally, the point of my inquiry is how the clause 'you were saying earlier' is joined to the 'that'(demonstrative pronoun) in “What's that you were saying earlier?” I would appreciate it if you explain your parsing way about the structure of this underlined part.
By the way, I parse that "What is that(=not only a demonstrative pronoun but also an antecedent) [(which) you were saying earlier=a relative clause with an objective pronoun - 'which' omitted]?“