Consolidating forests expertise needed

When you migrate groups, users and computers to forest b users will be logging to forest b domain controllers.  Also during migration computers will be disjointed from domain/forest a and joined to forest b. So depending on your link speed latency it might be good idea to setup domain controller from domain b in location a as it may improve login process. Also during migration you have option to migrate users/groups SIDs to new domain (So called SID history). When user from new domain b will try to access resources from old domain a it will simply provide its SID from that domain and automatically granted or denied access based on previous configuration.
The whole process of preparing and executing migration using ADMT is well described in series of articles linked below:
https://blog.thesysadmins.co.uk/admt-series-1-preparing-active-directory.html

Any details on what happens to exchange mailboxes if users are moved