Transunion is the largest of them, but I know one of the banks exchanges information with Compuscan & Experian. IIRC, the FirstRand group (FNB, Webank) makes use of Transunion and another bureau. Nedbank might make use of Experian, but blAbsa and Standard Bank, no idea.
All NCR registered credit providers (Banks) need to do some form of credit assessment as well as an affordability assessment. They calculate your risk profile based on credit bureaus information (number of accounts, judgements, accounts in arrears, fraud status, etc.) and also gets outstanding debt balances and instalment values for use in the affordability assessment. So, although the Banks do not use every single bureau out there (they pay for every enquiry), they need to at least make use of some of them.
As mentioned, Capitec doesn't offer asset financing, but rather personal (unsecured) loans. By default, an unsecured loan will have a higher interest rate; it's unsecured, i.e. there's no collateral, so they need to manage their risk somehow.
The higher your risk profile, the more you pay. The same principles are used in the insurance industries (life, asset, health).
The NCR is very strict when it comes to compliance and I know that they ever so often perform compliance audits at the service providers. Non-compliance is reported and the provider is fined. African Bank had to fork out a R250mil fine not so long ago for reckless lending/over extending clients (like buying a R16mil house on a R20k salary). If you are a bank, the NCR reports you to SARB, and you do not want to get on the bad side of them as you could lose your banking licence (something which is apparently extremely hard to get in the first place).