I think Proton Pass just stores one key for all devices? Not even sure! But it does work anywhere without the experience you had: I go to a website I have saved, it pops up, I click and am logged in.

Not sure if Proton does the device specific stuff under the hood (and hides it well), or if they are abusing the system by simply sharing the private key over all devices? (That is misuse right? Idk, I had the same experience with BitWarden). The keys should be device specific right? That's the 2fa replacing magic.

I too, have no idea. And I too am a bit disappointed it is so difficult to understand what happens. I do believe I can just export the keys and import somewhere else (i.e. Proton <-> BitWarden), which would suggest one passkey per account... Hmmm... Also, I believe it's just Google and Apple that try to make this a walled garden, it wasn't designed to be like that.