How can I receive a reimbursement done by Temu that hasn’t arrive to my account?

If the refund was through Apple it may take awhile. See the following from Check the status of a refund for apps or content that you bought from Apple - Apple Support

When will I get the money back?

If your refund is approved, the funds are applied to the same payment method that you used to buy the item. Here's how long it takes for a refund to appear on your account or statement:

Store credit—It might take up to 48 hours to see the refund in your Apple Account balance.

Mobile phone billing—It might take up to 60 days for the statement to show the refund. Processing time might be affected by your carrier. You can contact your carrier for an update.

Credit and debit cards, Apple Pay, Apple Cash, and all other payment methods—It might take up to 30 days for the statement to show the refund. If you don't see the refund after 30 days, contact your financial institution.

How does this involve Apple? Did you use Apple Pay?

Apple is not involved in refunds. When a merchant approves a refund, the funds go from the merchants bank account to your payment method you used to purchase the items and complete your Temu order.

If you haven’t received the funds, either your bank is holding the funds or Temu has not initiated a refund.

This is primarily a user-to-user support forum; other Apple users like you.

To contact Apple, see: Contact - Official Apple Support