A sales manager at AW Computing has created a contact record but is missing some of the information to complete the record. The organization-wide…

The user who creates a record is its owner and, by default, has full Create, Read, Update, Delete (CRUD) access. Therefore, the sales manager can edit the contact they created. Additionally, a user with the System Administrator profile has the "Modify All Data" permission, which grants them the ability to edit any record in the organization, regardless of sharing settings or ownership. The organization-wide default (OWD) settings for Account and Contact do not override the explicit permissions of the record owner or a System Administrator.

A. Users above the owner in the role hierarchy can gain edit access if "Grant Access Using Hierarchies" is enabled, but this is not guaranteed and isn't the most complete answer.
B. The OWD for Accounts is "Public Read Only," which prevents all users from editing accounts they don't own, and by extension, the contacts on those accounts.
C. Access is granted up the role hierarchy, not down. Users below the owner in the hierarchy do not inherit access to the owner's records.

1. Salesforce Help, Who Can See My Records?. This document explains that record owners can always view and edit their own records. See the section on "Record-Level Access".

2. Salesforce Help, Standard Profiles. The System Administrator profile description explicitly lists the "Modify All Data" permission, granting universal edit access.

3. Salesforce Help, Organization-Wide Sharing Defaults. This document explains that "Controlled by Parent" means a record's visibility is tied to the parent record's visibility.