Lemmy.world is currently the most popular large instance, Lemmy.today is another one. There are A LOT of instances to pick from if you’d rather connect to one closer to your region or attuned to your personal tastes, and they can all access posts from other instances so if you think an instance is run by people you can trust then feel free to make an account there.

To add onto Pug Jesus’ explanation, each instance is a self-hosted server separate from all the others. That means they can potentially be removed or re-added to the fediverse at any moment, they all follow their own rules so the only fediverse-wide restrictions are that the instance isn’t so insufferable that every other instance blocks them or defederates from them.

EDIT: A list of known servers HERE