Salesforce Facts by Abhishek

When creating a case from a community user, the ContactId is auto-populated with the contact associated with the logged in community user. The AccountId field is also auto-populated with the related account record.

Guest users cannot be the record owners. So, in order to set the owner for the records created by guest users we can specify a default record owner in Community settings. Reference:  https://help.salesforce.com/s/articleView?id=sf.networks_assign_guest_records.htm&type=5 Attached is the screenshot.