mongodb connecting to admin database but there is no admin database code example

Example 1: mongodb create admin user for all databases

use admin
db.createUser({ user: "mongoadmin" , pwd: "mongoadmin", roles: ["userAdminAnyDatabase", "dbAdminAnyDatabase", "readWriteAnyDatabase"]})

Example 2: mongodb add admin user

use admin
db.createUser({
  user: "username",
  pwd: "password",
  roles: [ "root" ]
})