Since Covid-19 is primarily airborne[1], the most important defenses are those that prevent you inhaling air that has been breathed out by infected people.
Outdoor spaces are much safer than indoor[2], and indoor spaces are much safer when well ventilated[3].
If you have to be indoors, then masks reduce risk by ~80%[5], with N95, KN95, and surgical masks being significantly more effective than cloth masks[4].
Rapid tests allow people to be screened for infectiousness before they enter a school or workplace - unlike diagnostic PCR tests, which do not give a result in time to prevent transmission[6].
Vaccines almost completely prevent death and severe disease, and are 70-95% effective against symptomatic infection (depending on the particular vaccine).