bank wants to know who our largest suppliers are, how should I fill that in

Supplier concentration risk, not the contracts themselves

The bank usually does not ask this out of curiosity about your purchasing advantage, but to assess concentration risk: how dependent is your business on one or a few suppliers, and what happens to your continuity if they fall away. This fits with the customer due diligence that banks are required to conduct. The answer that is usually requested is a general description: supplier name or type, country, and what proportion of your purchases come from them. Rarely does the bank want a complete supplier list with contract terms — that is more detail than the purpose justifies for most questionnaires.

If you do not have this overview ready, an estimate in broad strokes is usually sufficient: "three suppliers together approximately 60% of purchases, mainly in Germany and the Netherlands" tells the bank what they need, without you sharing business-sensitive contract details. The same question comes back in different wording at the average payment term to suppliers — banks often combine dependency and payment behaviour in a single risk picture. Note: this is a different question from that about the ultimate beneficial owners of your own business, where you can look at in case of doubt how you complete a bank's UBO question — suppliers and UBOs are sometimes confused in portals because they are in the same section "chain risk".

Where this comes from: customer due diligence and financial statements

The basis is the customer due diligence from the Anti-Money Laundering Act and Countering the Financing of Terrorism Act: the bank must assess the risks of the business relationship, and supplier dependency is part of that. Insofar as a supplier concentration is already recorded somewhere, it is usually implicit in the notes to the financial statements — which is why some portals place this shortly after the question about whether the financial statements have been filed place. A separate statutory publication obligation for supplier names themselves does not exist; this is purely a risk assessment by the bank, not an obligation under the Commercial Register Act.