Connect to localhost with admin user succeeds but encounters "Authentication failed" when trying to connect localhost/db, how do I do?

Question

I'm struggling to make a successful connection to a mongodb.

Here is what I did.

I setup the server following the instructions in some tutorials.

I ran this command from terminal

mongo 'mongodb://AdminSammy:test56789@localhost'

and got connection successfully.

AdminSammy is a user with Admin privilege, so, I created a database myshinynewdb and a table successfully with the user, following a doc.

what I'm confused about is

both these 2 commands

mongo 'mongodb://localhost/test'
mongo 'mongodb://AdminSammy:test56789@localhost'

can get connection successfully

whereas this one

mongo 'mongodb://AdminSammy:test56789@localhost/test'

cases

Error: Authentication failed.

Is it caused by the following config from the tutorial I mentioned at the beginning?

roles: [ { role: "userAdminAnyDatabase", db: "admin" }, "readWriteAnyDatabase" ]

How do I fix it?