Each car model has particular figures allowed for it. later models might have more stringent limits, but my point was a good running 23 year old car is already pretty close to zero, don't think innovation has reduced emissions much beyond that. Except hybrid, and shutting off while stopped but only for drivers stopped in traffic.
They do care most about equipment, not pollution. Also paperwork. One dealer who brought non-California cars into the state and installed emissions equipment so they passed tailpipe test was heavily fined. The cars didn't pollute, but state had not authorized those models.
When cars fail, owners may be given a grace period to keep using them, an income-based subsidy to repair, cash to retire. Often replacement equipment is needed, sometimes an overhaul.
The smog check I posted shows at 25 mph on dyno, hydrocarbons must be under 49 ppm to pass, average for my models tests 7 ppm, my car tested 2 ppm. Civic at the time it was 12 years old, 165k miles.
Sometimes I've had high emissions, corrected by replacing ignition wires or vacuum hoses. One time the Sable ECU gave error low efficiency of catalytic converter. It watches cycles of O2, loads up the cat with extra O2 to do its job. I replaced Cat and O2 sensors and it ran clean. (Previous overheating damaged fuel injectors, and bad mixture resulted in premature cat failure.)
California has ceased tailpipe tests for 2000 model and newer cars, just checks equipment (including exact model number of catalytic converter, ask me how I know), and reads ECU for codes indicating fault. The ECU checks O2 levels which relate to Cat (with O2 sensors after as well as before, it can see cat performance which relates to hydrocarbons). But it can't actually measure those, nor CO or NOx.
The old cars are allowed to continue being operated. A large fleet that will gradually be retired. Commercial trucks and construction equipment, older models are made illegal unless engines replaced. Yard equipment, there are noise ordinances and future sales of ICE models will be banned.
No need for riots, but money changes hands and smog certificates get issued. Stings are run to catch those shops.