Please find the below article. It has pre and post migration checklists items and how to get ready for the org migration. This helps to prepare for the org migration.

https://help.salesforce.com/articleView?language=en_US&type=1&mode=1&id=000334938

 For Hard-Coded Reference,

1. Check Remote Site Settings.

2. Check Apex Class, Visualforce page, Visualforce Component, Aura Component, Lightning Web Component.

3. Check Named Credentials.

4. Check Connected Apps.

5. Check Email Templates(It may use Images stored in Salesforce).