How do redemption codes work in Apple Business Manager when NOT using MDM The small dev company that I work for has been contracted to build an app for client.  This client is using Apple Business Manager and wants us to publish to it.

I have built the app and deployed it for review to the Apple Connect.  It passed and I see it is now in the client's Apple Business Manager account.

Here is where I am confused.  Our client DOES NOT have a MDM. How can I use Apple Business Manager in this case?  

From the apple site (https://developer.apple.com/business/custom-apps/), it says "Alternatively, organizations can choose to provide redemption codes to authorized users to download the app on the App Store.".

How do redemption codes work?
How does our client use redemption codes to download our app from the app store?

Our client simply wants its employees (1000+ employees) to download the app that we built (whitelabelled to the client obviously). 

Does the client have to add each of its users device UIDD's to its ABM? (which would be ridiculous)!  

How do these VPP codes work then? 

How do they redeem these codes on the Apple App Store, and therefore install the app on their device?
I am also using Apple Business Manager for my company but we have an MDM also with it. However, on your question, I think you have to go through the following steps:


Log in to your Apple Developer Account connect your App to the client’s Apple Business Manager account by adding their Organization ID and Company Name for distribution. See the highlighted blue area below.





This connection is immediate. Once your App is connected to the client’s Apple Business Manager Account, it will be visible under the Custom App section of the ABM account.
By selecting that custom app, you can generate 1-time use redemption codes and links for employees to download your app.




Once the codes are created, you can download the list.



You can use these codes and proprietary processes to enable your client’s employees to access your application.