GUYS: when you text "what's up"and your response?

now multiple times this guy I like (and I think likes me) has texted me first something like "Yo What's up?" "or "Hey what's up?" of once just "Hey!". So then I typically write something back like "Nm Hbu?" or if something new actually is up I'll tell him. So he'll write back and comment. But as far as when I ask him what's up all he ever says is something like "Just chillin' nothing fancy" or something like that. Why does he just text me this but when I ask what's up with he all he says is "just chillin?" and what is the meaning of saying "nothing fancy" ?
GUYS: when you text "what's up"and your response?
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