References:
https://docs.microsoft.com/en-us/office365/securitycompliance/faqimporting-pst-files-to-office-365 Explanation:
You can use the Office 365 Import Service to bulk-import PST files to Office 365 mailboxes.
When you use the network upload method to import PST files, you upload them to an Azure blob container named ingestiondata. If there are no import jobs in progress on the Import page in the Security & Compliance Center), then all PST files in the ingestiondata container in Azure are deleted 30 days after the most recent import job was created in the Security & Compliance Center.
Reference:
https://docs.microsoft.com/en-us/office365/securitycompliance/faqimporting-pst-files-to-office-365