You can provide the other companies with a copy of the package for them to release to their users, or you may consider publishing the app through the Play/App Store and allowing them to assign it to their devices from there. (I only manage Android devices, so other Apple options may exist.)
To keep things private, I would opt for distributing a copy of the package to their MDM team during each release cycle.
From an Apple perspective, I recommend looking into setting up the Application as a Custom App. Managing iOS Custom Apps: VMware Workspace ONE Operational Tutorial
Basically, by doing so, you can securely share the app to your partners/customers, but in a way that they can easily add them into their own MDM solution. Additionally, with the app in the Custom App store, it gains the same benefits as any normal app from the perspective of global CDN distribution and macOS Caching Services.