Hi,
With a OneDrive for Business account (with the company domain like @fakecompany.onmicrosoft.com), the global admin of the company (your old tenant) could check your files anytime with Microsoft365 admin center. If the boss and the admins are not in the same tenant of yours, they will not have the permission to access your OneDrive for Business.
You can refer to this similar thread:
For the sake of personal privacy and security, it is recommended that you use a non-company account to save your private important information and documents.
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