Google selects the extract itself from pages already ranking on the first page, choosing the passage that answers the question most plainly. A paragraph of forty to sixty words placed right under a heading that repeats the question, a numbered list for a how-to, a table for a comparison: those are the formats it lifts.
The snippet brings visibility and a share of the clicks, and it was the first form of answer engine before AI Overviews. The writing that wins snippets (the answer first, in a self-contained passage) is the same writing that gets quoted by assistants.