actually - it’s the server admins who pay for the moderation, not the users. And the fee is calculated by how many active users are on the instance, not how much work they generate.
So: Bob runs a server. They invite 20 of their friends to it, and everything is peachy. When these 20 get into an argument, or Alice posts an inappropriate picture, Bob is on it. With 20 users, it’s easy to do in ones free time.
Now, those friends are so excited about this, that they in term each invite 20 more users. So suddenly Bob has 400 users on his server, and a month later, it’s 2000.
Handling the moderation needs for 2000 users is too much work for Bob. So Bob turns to Darcy and pays a monthly fee to have their instance moderated. Bob can either pay this fee on their own (maybe they are a feeling generous, or have the extra income), or they can collect donations from the users. (Darcy will offer easy interfaces to Patreon or PayPal for this) Maybe they hit up the local hardware store to get a static “this instance is supported by DIY Haven!” banner somewhere.
(Bob might be tempted to add in automated banner advertising or worse. But that would get them into trouble with the Darcy moderation team terms of service, and the instance won’t be eligible for moderation anymore.)
We at Darcy strongly believe in paying the moderators, and paying them well. This is serious work, that needs serious training and also serious support. We’ve all read the reports out of Cognizant and similar outfits. Don’t be like that!