Plaid
Plaid is the service that connects your bank account to a financial app, so the app can verify the account and debit it without ever receiving your banking password.
When you link a bank account, you sign in to your bank inside Plaid's own window. The app you are linking to does not see those credentials and never stores them; what it receives is a token that authorises access to that one account.
This is why linking through Plaid is safer than typing an account and routing number into a form: the account is proven to be yours at the moment you link it, by your own bank.