Hi @unknown_25,
Graph API (MS Graph site object with "LastModifiedDateTime" property) is same with Last Activity in the Admin Center. This properties gets data from "Last activity” field via Admin Center GUI. The reports typically become available within 48 hours, but might sometimes take several days to become available. Here is the document for reference
And Web.LastItemModifiedDate is similar with Last Activity in the Admin Center. The only difference is Web.LastItemModifiedDate won't triggle when update m365 group membership via Teams by user.
Here is the difference between LastItemModifiedDate and Graph Last Modified DateTime
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