Think floor, not ceiling: if Account is Private, a sharing rule can grant extra read access; if Account is Public Read/Write, you cannot use a rule to hide rows from peers — you’d use other tools (profiles, FLS, etc.) for field-level control.
Most restrictive: users see owned records + sharing grants. Build up with rules.
Compare another object
Contact OWD (often more open than Account):
Real admins align Contact/Account OWD so related records stay coherent — this is a teaching sandbox only.
// OWD lives in Setup → Sharing Settings // It is not an Apex keyword — it shapes who can see rows before Apex runs.