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 Central auth on MediaWiki 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>