"Office 365 Tenant to Tenant Migration manual procedure is a process that allows organizations to move their data from one Office 365 tenant environment to another. This process can be used for migrating user mailboxes between tenants without the need for third-party tools or services. The steps involved in this migration include planning and preparation, setting up the target tenant environment, creating user accounts in both tenants if needed, running synchronization using PowerShell commands as well as the configuration of domains and DNS settings. Once all these steps have been completed successfully then the actual migration will take place which includes copying data from the source tenant into the destination tenant followed by validating results before finally decommissioning the old tenancy at the last step.
To read the complete procedure, visit the below article.
https://www.cloudbik.com/resources/blog/tenant-to-tenant-migration-office-365/