We cannot restrict Global Admins from gaining access to a Site Collection.
“Global admins and SharePoint admins don't have automatic access to all sites and each user's OneDrive, but they can give themselves access to any site or OneDrive. They can also use Microsoft PowerShell to manage SharePoint and OneDrive.”
You could refer to this article About the SharePoint admin role in Microsoft 365.
Similar issue for your reference:
Can i prevent global admins or other sharepoint admins from accessing our site collections
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