Where can I find basic fundamental information regarding the debit card and ATM functions?

Try this, it's very fundamental:
How Stuff Works

Just for the general information on the hows and whys, the ABA has excellant materials via the ABA website, training materials for bankers. The ABA Principles of Banking textbook is another good source.
If you have ATM and POS services at your operation the supervisors of your operations will be a good source of information on the how, when, what, and where. Your VISA/MasterCARD administrator will be another source.