[mbwgh]

My (uninformed and probably misguided) idea was that there was a host DNS service (responsible e.g. for resolving local domain names) which would cause Windows itself to trigger some rule when 'web.whatsapp.com' is encountered.

But yeah, the PWA thing seems more plausible, even though I was not aware of any install prompt or similar.

I would need to read up on PWA, and there seems to be a LOT unfortunately.