If you have two clients - andy andrews and andrea also - the database needs a unique id (identifier)
Andy would be "AA01" and Andrea would be "AA02".
In the Invoice tab or layout, there would be a field that would need to match the clientID.
Once "AA01" is entered into that field, Andy's address information would fill in.
Same with Products, they have a unique ID number.
On the Invoice on line number one, once the Product ID is entered, the cost, description and such would fill in.