I would like a novel in which the MC gets rich and tries to create a millionaire or billionaire empire (it has to be us modern times)
Here: https://www.novelupdates.com/series/i-have-a-mansion-in-the-post-apocalyptic-world/ It's good, has what you want, but also a mix of sci-fi and post-apoc. The MC's 'empire' spans both timelines too, which is cool in and of itself. Haha.
https://www.novelupdates.com/series/extraordinary-genius/ https://www.novelupdates.com/series/monster-factory/
Very Pure and Ambiguous: The Prequel Returning from the Immortal World I’m in Hollywood HacKer I Have a Mansion in the Post-apocalyptic World