Where is the list of attached databases stored?
I don't see a table in master.mdf ... where the names of the other attached databases are stored.


That would be sys.databases. Running sp_helptext 'sys.databases' will quickly reveal what is the catalog table backing it:

CREATE VIEW sys.databases AS
    SELECT d.name, d.id AS database_id,
...
FROM sys.sysdbreg d 
...


To query sys.sysdbreg see Using a Dedicated Administrator Connection.