Where are SQL Server catalog views persisted? It seems that sys catalog views can be found under every database, either built in or user created. Are they all actual view definitions created under each of the database or just "links" to somewhere?
Thanks
Catalog views do not provide access to replication, SQL Server Agent, or backup metadata.  Those are available in MSDB only.

Reference:

https://docs.microsoft.com/en-us/sql/relational-databases/databases/resource-database

https://technet.microsoft.com/en-us/library/ms179503(v=sql.105).aspx


  The Resource database is a read-only database that contains all the
  system objects that are included with SQL Server. SQL Server system
  objects, such as sys.objects, are physically persisted in the Resource
  database, but they logically appear in the sys schema of every
  database. The Resource database does not contain user data or user
  metadata.
  
  The Resource database makes upgrading to a new version of SQL Server
  an easier and faster procedure. In earlier versions of SQL Server,
  upgrading required dropping and creating system objects. Because the
  Resource database file contains all system objects, an upgrade is now
  accomplished simply by copying the single Resource database file to
  the local server.
  
  The physical file names of the Resource database are
  mssqlsystemresource.mdf and mssqlsystemresource.ldf. These files are
  located in :\Program Files\Microsoft SQL
  Server\MSSQL.\MSSQL\Binn\ and should not be
  moved. Each instance of SQL Server has one and only one associated
  mssqlsystemresource.mdf file, and instances do not share this file.
  
  System base tables are the underlying tables that actually store the
  metadata for a specific database. The master database is special in
  this respect because it contains some additional tables that are not
  found in any of the other databases. These tables contain persisted
  metadata that has server-wide scope. The Resource database contains
  the same base table schema as any other non-master database.