r721 parent
There are some tricks which work for different websites - for example, for NYT it's enough to manually clear nytimes.com cookies, FT used to work after click from twitter/x and so on. So I guess there is some set of heuristics.
It seems that archive.is often has the full article for sites that are completely paywalled to every non-paying visitor: no cookie-driven freebies, nothing.
Publicly revealing everything they are doing would be a strategically bad idea, obviously.
It's not inconceivable that they actually pay for access to some of the sites; it wouldn't be surprising.