Selfhosted

Most of this comment was my own speculation based on the details they've shared publicly. The details I know of publicly are:

• The seem to be profitable. Or at least in a relatively sustainable place; they talk about profit a lot, but usually in terms of how the "profit" is split between creators. I forget, maybe the Wendover "history of Nebula" video from a while back talked more specifically about profitability?
• They're choosing not to take outside investment. This is something the CEO, Dave Wiskus, talked about particularly with respect to the Lifetime subscriptions, describing those as their option for building up the sort of large amounts of cash that they might otherwise have gone to outside investment for, in order to fund bigger projects
• The fact that they are, quite visibly, expanding their range of content

The rest was me speculating about how the business model would seem to work based on those factors plus my limited, layperson's, understanding of their industry.