I do have a lookup in a SharePoint list . The list contains each category will have an approver's name and email. The user won't know who will be the approver to choose in drop down. So in Power apps, once the user choose the category, an email will be sent to the approver. Let's say if one of the approver resigns and i edit the lookup name and the email, the existing list also do updates to the latest lookup. How do I avoid this ?
There is no way to avoid this. Lookup columns store the ID of the item you have referred and not the actual value from the other list.
For example, I have a list A with columns Department & Approver and another list B which has columns - Request, Approver (Lookup from the list A). You add an item in list B and select an approver. What actually is stored in this column is the ID of the item in list A with selected approver. So whenever you change anything in this item in List A, the lookup value will also change. If approver is updated in List A, approver will also be updated in List B.
This is one of the benefits of using lookup columns in SharePoint. If you want a column where if I change the value of it and it should be changed in the child list where it is referred, only then you use a lookup column.
To preserve the values of old employees you can use choice columns fro approvers or single line text which will store email of the approver.