Internal users explanations:

The operator uses the following internal DB users:

• operator - the user that charm.py uses to manage database/cluster.
• monitor - the user for COS integration.
• backup - the user to perform/list/restore backups.

The full list of internal users is available in charm source code. The full dump of internal users (on the newly installed charm):

ongodb-k8s [primary] admin> db.system.users.find()
[
  {
    _id: 'admin.operator',
    userId: new UUID("f4466ca5-7640-4af3-b9eb-6b5a2313696a"),
    user: 'operator',
    db: 'admin',
    credentials: {
      'SCRAM-SHA-256': {
        iterationCount: 15000,
        salt: 'F6HeW8j0Vseza/r3anU6cHBy+Uiu1f8nmMn8TA==',
        storedKey: 'yHi6aWAs0blfXiePgh9nkzXw/Vr0JBhBUV0VmXysx+s=',
        serverKey: 'sJpXiPUrEEV9qlf8ZFng0zolN0Ii3MXKnta1mvaVTEI='
      }
    },
    roles: [
      { role: 'readWriteAnyDatabase', db: 'admin' },
      { role: 'clusterAdmin', db: 'admin' },
      { role: 'userAdminAnyDatabase', db: 'admin' }
    ]
  },
  {
    _id: 'admin.backup',
    userId: new UUID("ece48d5e-985a-4863-8660-8fa2e1a0aa7d"),
    user: 'backup',
    db: 'admin',
    credentials: {
      'SCRAM-SHA-256': {
        iterationCount: 15000,
        salt: 'QktAJGMGslnfpHg6p/Dd7eGlYb8uzBdBsCEiwg==',
        storedKey: '/abOk7NijI9u7rf8leRcnpBYImGshmGW7BsXwVpFYrQ=',
        serverKey: 'QWYXFJm1GhqrGjvqZTAtdKGitL+8B1LNIF3OmkiYyiU='
      }
    },
    roles: [
      { role: 'readWrite', db: 'admin' },
      { role: 'clusterMonitor', db: 'admin' },
      { role: 'backup', db: 'admin' },
      { role: 'pbmAnyAction', db: 'admin' },
      { role: 'restore', db: 'admin' }
    ]
  },
  {
    _id: 'admin.monitor',
    userId: new UUID("59560831-741d-4417-938a-68056bc80c47"),
    user: 'monitor',
    db: 'admin',
    credentials: {
      'SCRAM-SHA-256': {
        iterationCount: 15000,
        salt: 'QElyLpAw5PFmc4juu6Nk94Hv/4H1RxQfDgVEyg==',
        storedKey: 'bW2wC8DheiNkSMdMtZcq149qE9hKIErgcJVtRRghhqA=',
        serverKey: 'AU271CqwEMTFDorYRPVgyU1J5SvNLIVFWAItw19TvOg='
      }
    },
    roles: [
      { role: 'clusterMonitor', db: 'admin' },
      { role: 'explainRole', db: 'admin' },
      { role: 'read', db: 'local' }
    ]
  }
]

Note: it is forbidden to use/manage described above users! They are dedicated to the operators logic! Please use data-integrator charm to generate/manage/remove an external credentials.