Setting up a sharing set to grant access based on the community member's contact record is a solution that can implement the requirement of enabling community members to access, create, and manage cases online. A sharing set is a feature that allows administrators to grant access to records for community users based on their user profile and a common field on the user and parent records, such as the contact or account fields. A sharing set can help ensure that community members can only see and edit their own cases or cases related to their contact or account. Verified References: : https://help.salesforce.com/s/articleView?id=sf.
networks_sharing_set.htm&type=5 : https://help.salesforce.com/s/articleView?id=sf.
networks_sharing_set_create.htm&type=5