One-time clog vs. a main-line problem
It depends on the cause. A one-time clog from something a tenant flushed is usually the tenant's to clear, but a toilet that repeatedly overflows or backs up often points to a main-line or sewer problem that is the landlord's responsibility.
A single clog from too much toilet paper or a flushed object is normal, and a tenant can often handle it with a plunger. But when a toilet backs up again and again — or backs up along with other drains — the real problem is usually deeper in the plumbing: a blocked main line, tree roots, or a failing sewer that the landlord must repair.
If plunging does not fix it and it keeps happening, treat it as a plumbing problem, not just a clog, and report it in writing. If your landlord blames you for a recurring backup that is really in the building's plumbing, a free review can help you sort out responsibility.
This is general information about California tenant rights, not legal advice. Every situation is different.