Apple Business Manager (ABM) and Apple School Manager (ASM) supportCustom Apps, which are distributed privately through the App Store. According to Apple's learning materials, Custom Apps are developed using standard App Store guidelines but are assigned specifically to organizations. Thedeveloper must explicitly assign the appto a particular ABM or ASM account by using the organization's ID. Once assigned, the app appears in the Apps and Books section, where licenses can be purchased or redeemed. IT admins do not submit apps through AppleCare; this is handled solely by the developer. This ensures apps are securely distributed only to intended organizations.
References:Apple Business Manager User Guide - "Buy and distribute custom apps."