Technically, The finance department needs to migrate their users from AD to AAD using AADC based on the finance OU, and need to enforce MFA use. This is conditional access policy. Employees also often get promotions and/or join other departments and when that occurs, the user's OU attribute will change when the admin puts the user in a new OU, and the dynamic group conditional access exception (OU= [Department Name Value]) will move the user to the appropriate dynamic group on next AADC delta sync.

https://docs.microsoft.com/en-us/azure/active-directory/enterprise-users/groups-dynamic-membership

https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/overview

https://docs.microsoft.com/en-us/azure/active-directory/authentication/howto-mfa-userstates