Hi @dasrp,
No, the SharePoint Online Administrators have all access to all contents in your tenant. The SharePoint Online Administrator role has the ability to override any restrictions that have been put in place, so it is important to only assign this role to individuals who are fully trusted.
In your scenario, I suggest you use teams/group admins instead of SharePoint admins. You can create several teams/groups for different companies and add different group/team owners for different companies.
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