Global user page
Every Miraheze wiki has a separate user database. Every time a logged-in reader visits one of the other wikis in the Miraheze constellation of wikis, an account is created on that wiki by MediaWiki's "CentralAuth" extension (see https://mediawiki.org/wiki/Extension:CentralAuth for advanced information about that). CentralAuth was designed to make it easy for one server cluster to run many wikis, but not have duplicate username/password pairs for each wiki.
The result: when users sign up for an account on a Miraheze-hosted wiki, the username/password database for that wiki is managed by Miraheze. The "central" wiki used by Miraheze is <https://login.miraheze.org>