I'm new to the fediverse but have a long history with FOSS and P2P. Realistically, the fediverse is not a money-making venture. It's a passion project to fill a gap in existing software offerings. The source code is hosted online (just confirmed by visiting kbin and Lemmy on GitHub). But the reason they can run without massive influxes of cash is because they are P2P in the resources they consume (fediverse style).
So donations are key. @ernest won't get rich on donations, but they can keep him supported while he develops and maintains kbin, and his experience with kbin could underpin and propel a very prosperous software engineering career, where his reputation precedes him. The passion project continues and the software remains in positive development. I've done the exact same thing in my career and I'm probably 5-10 years ahead of where I'd otherwise be because of my FOSS passion project.
Since the fediverse is a peer-to-peer structure with freely available foundational software, anybody can pitch in resources. Anybody with a computer, an Internet connection, and some sysadmin skills can run a fediverse server. Mileage will vary based on connection speed, hard drive space, etc, but it's completely doable. Lemmy/kbin servers are probably going to be the next most popular service that homelabbers start hosting, and peak-demand can be offset with scalable solutions like virtual servers (VPS offerings like AWS, but these cost more immediate $).
So knowing that, hopefully your question is answered. Short story is the fediverse follows the peer-to-peer (P2P) backbone that torrenting and other piracy methods used (e.g. Napster or Soulseek, but without a central server!) and similar to Tor as well. I specifically name those networks because P2P networking has been incredibly resilient for decades (except Napster, whose central server was shut down).