Hollo is a federated single-user microblogging software powered by Fedify. Although it is for single-user, it is designed to be federated through ActivityPub, which means that you can follow and be followed by other users from other instances, even from other software that supports ActivityPub like Mastodon, Misskey, and so on.
Hollo does not have its own web interface. Instead, it implements Mastodon-compatible APIs so that you can integrate it with the most of the existing Mastodon clients.