(1) This is a house. Jack built this house.
(2) This is the house that Jack built.
I don’t think people say (1) in usual conversation. People will not show the house and say, “This is a house,” unless it does not look like a house, because everybody can tell a house from other things. Moreover, English learners wonder why “a house” in (1) changes to “the house” when “This is a house.” is linked to “Jack built this house.” in sentence (2).
The following sentence order can occur.
(a) Jack built a house. This is the house.
Then, isn’t it possible to think that the second sentence (“This is the house” in the above case) can link to the first sentence (“Jack built a house.”) to make the sentence, “This is the house that Jack built.”?