I keep hearing a lot of talk about Passkeys, but I haven't yet seen any web site, service, platform, or cloud where I could actually use one.

Has anyone used these in the field for a major, popular site or product? What was it?

More importantly: What if I have a Windows laptop and an iPhone? Can I transfer a Passkey from my "only Apple device" to something else as a backup, or will these be locked to one vendor?