The above is a broad question. But to summarize:
A good data migration strategy and access to data management tools are what you need. Microsoft has provided a lot of tools to leverage.
Data management framework in D365, along with data management tools in LCS [lifecycle services], can be looked into to help decide your data migration strategy.
In the D365 data management framework, you can use data entities, data templates, data packages, copy legal entity functionality, [when dealing with multiple entities that share data].
D365 data migration capability is extended to the use of LCS [Lifecycle services] and how you can leverage it to migrate the data within D365. E.g. data policies [for cross-company data sharing], and through user-defined data templates that can be sequentially loaded in an environment, etc.
The below links might help you understand the D365 data management framework and various tools available in LCS :
docs.microsoft.com/.../data-entities-data-packages
docs.microsoft.com/.../cross-company-data-sharing
