I haven't heard of "Premium" encryption specifically. I do recall reading that 365 Business had better encryption protection than 365 consumer licenses. I don't remember the specifics.
Take a look at this page for the MS propaganda on 365 encryption
https://docs.microsoft.com/en-us/office365/securitycompliance/encryption
This is an older file, but it may give you some key words or clues you can use
File Protection Solutions in Office 365:
http://ligman.me/2v0u0jd MSFT_cloud_architecture_O365 file protection.pdf
Check out the MS Virtual Academy (may now be Microsoft Learn), they have/had a course, "Encryption in Office 365". You may be able to view it during a short free trial period.