Migration Service
SharePoint offers a migration service in SharePoint admin centre. This is know as SharePoint Migration Tool (SPMT). WHAT A NAME! You can move other third party cloud files or on premise files into SharePoint using this tool. There are other third party tools that can handle this with a more bespoke approach such as ShareGate.
Backend Migration
Firstly, I am not a genius so some of this may be be false. Sue me if you want.
Its obviously built on Microsoft azure based infrastructure but uses a bunch of backend components and services.​
The first thing it does is uses data extraction to read the source. (All the files and metadata). It temporarily stores the data in azure blob storage staging and then to push it from azure to SharePoint online it uses Microsoft graph API and SharePoint Online CSOM. It does also use Azure AD Authentication for ...hmmm... well Authenticating.
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Security
Microsoft ensures that data is encrypted from both ends (I hope so). It uses AES-256 Encryption during the transfer and within azure storage.
As mentioned before OAuth means only authorized users can start migrations.
Microsoft do comply with all government requirements such as GDPR.
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Ready to Migrate?
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The actual doing of the migration is simple and mostly a waiting game depending on size.
There will be another page on SharePoint best practices to look for!
To migrate, simply head to the SharePoint admin centre (make sure you have an account with appropriate permissions) and find the migration section.
The interface will run you through everything, as long as you have permissions for source and destination you will be fine.
Make sure if you are moving data from on premise you have enough free storage on the file server for the cache needed (its around 150gb)
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