Apple Business Manager and Apple Developer Program My team developed custom application for a company, and now we are trying to deploy the application to the employee owned devices. We decided to use the Apple Business Manager and some MDM solution for managing the devices.
1.Using the Apple Developer Program is the only way to make the application available on the Apple Business Manager account for further distribution, is this correct?
2.Once the application is distributed, from the Apple Developer Account to the Apple Business Manager, in case there are no updates needed for the application, there is no need to renew the Apple Developer account and pay the fee. Meaning the application will still be available for distribution and using on the Apple Business Manager Account. Is this also correct?
No - there are multiple avenues for distribution. MDM can assist with many and it depends on the MDM and the app entitlements and where and how you upload the app.
When a developer account expires, you lose control and can’t renew signing certificates. Currently, Apple doesn’t pull these apps, but there’s  no reason they couldn’t in that case from my reading of our contracts in the US.

With B2B custom store, public store with private SKU pricing and more, you have diverse options here.

https://www.apple.com/legal/enterprise/apple-business-manager/abm-us.pdf