The SARS-CoV-2 virus having only been circulating among humans for 20-21 months (IMO, it was a gain-of-function virus, leaked from the WVI lab, though I'm open to better, more complete, info), and not recognized as something new for a couple of months, there's a LOT we don't know about it. As you suggest, Texacon, SARS-CoV-2 being a coronavirus and other coronavirus types being in circulation among humans for centuries or millennia, some people's immune systems were able to recognize and to some degree fight the SARS-CoV-2 virus.
Severity of infection is affected by multiple factors, many/most not yet known. Closeness and duration of exposure are relevant. Recent vaccinations of another kind may be indirectly relevant, especially in children (adjuvants commonly paired with many vaccines to improve vaccines' effects, allowing smaller doses, stimulate recipients' immune system in a general way). A wide variety of health conditions may be relevant. It is known that there are many people "out there" who had asymptomatic infections or who had symptoms so mild that they did not recognize they had Covid rather than an allergy or cold. The size of this sub-population among people who have immunity from recovery is really not known, and of course, some got vaccinated. At the same time, some/many un-vaccinated people have immunity from Covid without knowing it.
The real world is messy and complicated. People who speak of the CDC's 48.6% fully vaccinated number as how many people in the US have meaningful immunity from Covid are uniformed and/or lying. Those who have only received one dose of a two-dose vaccine have some immunity, possibly as good as the single dose J&J vaccine. People who have immunity from recovery - those who know they recovered and those who do not - are ignored, even by government "health" "experts"; since those "experts" know better, they are being dishonest. Then there's people who have some degree of immunity from exposure to other coronaviruses.