How aircraft cabin air systems work and reduce infection likelihood
Pall Corporation’s website has a good description of how aircraft cabin air systems work and in particular the 19 min video on it which describes the pattern of airflow and how HEPA filters remove contaminants down to very small sizes.
The regulation variously described as 0.28m3/min, 0.25kg/min, 0.55lb/min is for large transport aircraft certified by EASA in Europe or the FAA in America, which are aircraft of greater than 19 seats or greater than 19,000lb maximum take-off weight. Smaller aircraft do not have a mandated ventilation flow and thus likely to have much simpler systems.
There are very few aircraft operating as passenger carrying that are greater than 25 years old, so the vast majority are equipped with HEPA filters. All Airbus aircraft have been equipped with HEPA filters since 1994.