Hi Emily!
Glad the explanation was helpful. To your question,
With global reference questions like main idea and purpose, do they always encompass an idea from each paragraph?
My only hesitation with answering the question in the affirmative is the "always." It seems safe in general as a strategy to expect each of the paragraphs to be encompassed within global reference question types like that.
But it often will not be as clear as a one-to-one correspondence in terms of an answer choice with three sentences for each of three paragraphs. Rather, you might read a sentence in the answer choices for such questions that seems clearly to refer to the first paragraph but then discover (perhaps after eliminating other answer choices) that it actually covers both the first and second paragraphs.
In addition, asking for the main idea or purpose is a more specific task, beyond just knowing the structure of the passage. For instance, the second paragraph might be full of examples. Knowing
why the author fills that paragraph with examples is different from merely describing it as such. Such examples might be used to embellish or demonstrate what was mentioned in the first paragraph. Or they might be used to contrast with the first paragraph.
Understanding the structure, but also why the author designs the passage in the way it is structured is important for identifying the main point. It might be something as direct as the final sentence of the first paragraph, or a single sentence raised in the concluding paragraph. In such cases there isn't necessarily a one-to-one correspondence with the answer choice to each paragraph, yet if a single sentence really captures what the author is doing, then it is possible that even that could be referring to all of the paragraphs.