Which process changes source mailbox to a linked mailbox??

I am migrating only https://100001.onl/ users and computers https://1921681254.mx/  from source to target domain https://happymod.site/ using QMM for Active Directory, the users have their mailboxes on Source Domain while I will migrate the users to the Target Domain.

At what stage or which process will convert the User Mailbox in the source domain to a linked mailbox, so that once the user is in Target domain will continue to have access to their mailbox? is ADPW that will do this?