A single overflowing fixture may be local; multiple low fixtures affected after any upstream use point farther downstream.
The City of Ridgecrest operates its collection system and provides channels for sewer issues. That public role does not prove a particular backup is city responsibility. A private branch, lateral, or public main can produce overlapping symptoms.
Keep this boundary: Do not promise reimbursement or assign fault. Ownership and causation require the exact line location and applicable local rules.