The short answer is that he is not anywhere close to a billionaire.
To the extent he has money (20 or 30 million), it appears to stem from his prior life as a tv star.
For example, at least two sites, TimiSsss News and Pledge Times mention that Zelenskyy would have money stashed in the Dresdner Bank Lateinamerika, a financial entity that does not even operate in Costa Rica.
Both publications claim that their sources of information are Telegram channels associated with Ukrainian opposition politicians and circles close to Russian President Vladimir Putin.