See Details of Finished Approvals
Although all the information about approvals is kept in the task/folder/project stream, it's also possible to come back to the selected decisions made regarding a task, folder, or project and see additional details in one place.
Navigate to the folder, project, or task that has details on the finished approval.
Open the task in Task view. If you select a folder or a project, open its info panel.
Click the Approvals (or Approved) 1 button on the panel above the description field.
Click on the approval for which you'd like to see details. 2

In the pop-up that opens you'll see:
Who started the approval process and when.
Who approved or rejected the approval.
Comments left regarding the decision.
If files were added to the approval, you can see details and comments by switching to the Files tab.