i think these studies are always observational studies - as its rare that you can do controlled experiment in humans where there are risks of negative side effects - for ethical reasons.
as you probably are aware - you can never absolutely prove causation via observational studies alone
to get the best evidence they can they tend to observational studies with large cohort size -
and back them up with placebo controlled trials in culture and animals etc
they are however usually able to do a good job of controlling for variables such as advancing age and other health conditions, lifestyle factors etc to give pretty robust results - even after these factors are accounted for.
i haven't dug into it much more than that as i don't take calcium as a supplement
there is a decent article here : https://chriskresser.com/calcium-supplements-why-you-should-think-twice/