hey, good question! tbh i don't think there's just *one* specific markup that guarantees an AI snippet. from what i've seen, it's usually a combo of things. schema markup (especially FAQ or HowTo schema) helps a ton.
but also, just having super clear, concise answers to common questions, good H1/H2 tags, and well-structured paragraphs. google's pretty good at figuring out context even without super specific markup if the content is clear. hope that helps a bit!