Universal Containers’ organization wide-defaults model is Private for the Account object.

With a private organization-wide default (OWD) model for the Account object, users do not have automatic access to related records they do not own. However, Salesforce provides implicit sharing, which ensures that the owner of a child record, such as an Opportunity, is granted Read access to the related parent Account. This implicit sharing does not extend to Edit or Create permissions unless additional sharing rules, roles, or explicit access mechanisms are configured. Therefore, the sales rep will have Read access to the Account associated with their owned Opportunities. (developer.salesforce.com)