Memento has no account, no login, and no server. Everything you capture is stored in your own Apple Reminders account — it stays yours.
Memento uses your iCloud Reminders account, protected by your Apple Account and encrypted in transit and on Apple's servers. Your items sync across your devices and stay readable in the Reminders app. Memento never sees them on a server of its own, because there isn't one.
Memento shows one account: iCloud when you're signed in, on-device otherwise. Lists from other accounts, like Google or Outlook, don't appear.
Deleted items are held so you can get them back. Open Recently Deleted from your Home screen.