@mamur I don't know how to see how many we are, but You see people from other domains (instances) because we're all connected and can see each other as a decentralized network. The different menus as I understand, someone correct me if I'm wrong:
Home - a mix between your instance "qoto.org" and the people you follow, which may be in other instances such as mastodon.social
Local timeline - Your instance timeline (qoto.org)
Federated timeline - A hell where all the different instances converge