When a Salesforce admin assigns User A as the owner of an opportunity related to User B's account, User A will have Read-Write access to the opportunity's Account and its related contact records. This is because of the implicit sharing rules that grant access to parent and child records based on the organization-wide default settings. Since the organization-wide default setting for Account is set to Private, users are unable to see each other's accounts by default. However, when a user owns an opportunity that is related to another user's account, the user gains Read-Write access to that account and its related contacts, regardless of the sharing settings for those objects. This ensures that the user can view and edit the account and contact information that is relevant to their opportunity.