If they have bitcoins, hold their bitcoins to get passive income with bitcoin only. They don't have to do any extra thing, just holding.
It is not passive income either because holding doesn't increase your bitcoin holdings in any way but in terms of fiat that may give returns.
Since OP is looking for passive income then better look for the traditional methods because in crypto which is not worth the risk like staking or casino bankroll investment due to the nature of price volatility and there is always risk of the platform becoming scam/hack.