Which field in database tells us about the ID of Resource to whom Timesheet is submitted for approval ??
No specific field because there is no specific approver (anyone who has approval rights can approve a timesheet).
From what I understand from your other posts, you have an Action Item based timesheet approval process, so YOU need to look at your Process that controls that approval and the logic within that Process will tell you your answer. It might be that you can determine from the relationship between the timesheet and created Action Items who the approver(s) were.
EDIT : you might also find some useful information in this thread ; Timesheet approver
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