The UML diagram generally shows the relationships between classes so the 'connection' is through the headers.
main() is there as an interface which relies on those connections being made beforehand. The interface can be console based, GUI, smartphone while the classes and their relationships remain.
A few unsolicited thoughts on your program:
Why not just have an Account class, where the interface (ie main() ) is the ATM? The methods for Account would be makeDeposit(), makeWithdrawal(), getBalance() etc.
The methods for an ATM if that was the way, would be aspects covering logons, passwords, displaying a menu, retrieving/displaying info for a particular account, not the whole bank database. BTW the Account queries the Bank (class) on that account_no, but again doesn't retrieve the whole database.