The Fediverse is essentially a collection of platforms (such as Lemmy, Mastodon, Pixelfed, and more). Each platform has a number of instances that are hosted separately, hence the decentralized nature of the Fediverse. For example, Lemmy is a platform, and Lemmy instances include Lemmy.world, Lemmy.ml, kbin.social, sh.itjust.works, and more. Others have posted links to lists of instances that you can look through.