https://biology.stackexchange.com/help/site-moderators This link tells you a bit about site moderators.
Moderators are volunteers, not paid. They are elected from the community by users. Every subject site on Stack Exchange has their own set of moderators. Biology has 4 of them right now, and we all have advanced education in various parts of biology, but it's not possible for any of us to be experts in everything biology-related; biology is a huge field. Some StackExchange staff, Community Managers, work a bit like "super moderators" to resolve disputes, set policy, and handle special cases with legal implications for the company.
All StackExchange users can participate in moderation by voting on posts, voting to close/reopen/delete, flagging material that is spam, objectionable, or against site policies. Users with more reputation can participate in more of these tasks.
Moderators tend to be elected from among people who are most interested in the community, so they may ask and especially answer a lot of questions, but they aren't expected or required to do so. Correlation, not causation.
Speaking personally, actually moderation takes very very little of my time. I spend far more time on answers themselves, which I also did before I was a moderator, and also on curating posts: voting to close, suggesting changes in comments, occasionally edits (but some of my co-moderators are far better at editing).
Very few tasks I do as a moderator can only be done as a moderator. The most common ones are addressing problematic users who are not here to participate earnestly, like anywhere on the internet - they may be present to spam, or post racism or conspiracy theories, or just cause trouble (troll).