This article[0] provides some details. Basically if you go through the lookup process on Apple's website and you don't have an existing D-U-N-S number, you can request one from D&B for free via Apple.

[0] https://support.pushpay.com/s/article/Acquire-your-D-U-N-S-n...

I don't know how obvious it is these days, but the default path through D&B's website is the terrible one. They will try to extract money from you and harass you forever. You had to find Apple's own embedded form for it by using their search and going through some flow.