How do you strike up a conversation with someone you don’t really know but are interested in?

I’m not into online dating so I’m talking about meeting guys in real life. For instance if I meet a guy while I’m volunteering, what should I say to signal that I’m interested in getting to know them more if I don’t know if they’re single? I obviously wouldn’t ask them out but what exactly should I say instead
How do you strike up a conversation with someone you don’t really know but are interested in?
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