The usage of "this" and "it" in a given example

I came across these sentences in Cambridge, and I do not understand which words "this " and "it" refer to. I have three questions that need your help as answers.

Here are they.
The example: "Put the butter, chocolate and sugar in a saucepan. Heat this over a low flame until it melts."
Question:
(1) "this" here refers to which word?
(2) "it" in that text is a dummy subject or a real subject?
(3) If both "this" and "it" imply a group of words before, why "this" is used first and" it" is used after that?

In this case, the word "this" is used to refer to the single and uncountable nouns. In this case, the uncountable nouns refer to chocolate, sugar, and butter in its original form. The use of "it" in the sentence cannot be considered a dummy subject because it refers to a specific subject in the sentence.

Also, a dummy subject is always presented in the form of a phrase such as " It is" and "It was". The phrase is oftentimes used to add words to a presentation, without actually adding context to the presentation. Therefore, it is a "dummy subject". "It" and "This" are often used to refer to previously mentioned references.

Consider "it" as the reference subject in a sentence. It is a direct or indirect object of a verb, normally the object of a preposition to an inanimate object.