I want to send an email to my users with the link to a Clappia App where all details are pre-filled from another App or database. The end-user will just review all details and click on one Single-selector - Approve/Reject. Is this possible?

Yes, that is feasible. You can send a custom URL with the submission id in the format company_name.clappia.com/app/AppID/submissions/$submissionID over email.
If the user has submitter or submissions edit access, he can click on the single selector to approve or reject the submission by clicking on this link. 
While configuring the app, under the edit submission workflow, a process must be configured using two If nodes for both the selections (Approved and Rejected) following the edit submission node, which will update the status of the particular submission on the app depending on the selection.

Workflow Setup:
IF Node: @selector_field="Approved"
Edit Submission Node: Source App: SameApp
                                           Filter Value: Submission ID - $submissionID
                                           Select Field Values: Submission Status: Approved

Help Guide Link for,
Status Configuration: https://help.clappia.com/help/app-settings/17
IF Node: https://help.clappia.com/help/workflows-condition/14
Edit Submission Node: https://help.clappia.com/help/workflows-edit-submission/18