D & B get most of their info directly by phone interviews.
They have a vast database on American and Canadian businesses.
They got their start in the 'rag trade' where suppliers and customers constantly get burned. See "Who'll Save The Tiger"...
You'd be surprised how much D&B can find out without even leaving their desk. Most of your distributors are already rating your payment record and reporting it to D&B automatically. That's what is of primary interest to creditors, anyway.
D&B may well call your landlord, if any, to see if they are happy. It is not uncommon for D&B to stop on by and visit to look over your facilities. Don't be surprised if they don't hit you up to be a subscriber.
The typical report sent to subscribers will show who the principals are, the legal form of business, its address, phone number, etc. It will also have a guestimate as to the size of the firm based on their own take. Any liens or litigation will be posted since D&B routinely logs all commercial litigation and bankruptcy filings.
It is not a cheap service for a small guy, but is deemed essential by bankers and big players.
Investigation is normally triggered by a subscriber inquiry. I was almost always amazed at the info D&B had on my customers.