You want to sync your data between app1 and app2. Is there a golden source?
If you open your entity in your domain model, you can check the system members like ‘createdDate’ and ‘changedDate’. You can use those attributes to know which record is new or updated.
In your microflow you can do a retrieve from database and than set a XPath contraint to receive all records created/updated last 24 hours for example. Use an export mapping to map your data to the structure for your PATCH endpoint.