In order to distribute custom apps to your employees via Apple Business, your company must first be registered with the corresponding Apple program. Many companies already have an account, which is used for Apple's Volume Purchase Program, through which apps are purchased from the App Store for distribution to managed devices.
Once the custom app is approved for your Apple Business account, you can select it in the Custom Apps section. There you can create redemption codes with a few clicks, which you can then distribute to your employees to download the app via the regular iOS App Store:
Requested redemption codes can be downloaded as an xlsx file from Apple Business after a few minutes. Each redemption code which is obtained via Apple Business for a custom app can be redeemed by exactly one Apple ID. The iOS App Store app will guide your employees through the process after redeeming a code.
Afterwards, all iOS devices on which the corresponding Apple ID is logged in will have access to the custom app. Your custom app can be easily downloaded via the "Purchases" section in the user profile of the App Store app.
Currently, it is not possible to redeem redemption codes in the iOS App Stores of any country. They are limited to the App Store of the country for which the company has been registered with Apple Business. This topic is discussed in the Apple developer community, e.g. see here for the latest:https://developer.apple.com/forums/thread/650957