Honest question, this isn't supposed to be a burn in any sense. But isn't it contradictory to want to know how and where your credentials are stored and use any credential management solution that isn't opensource and selfhosteable?
1Password is neither opensource nor self-hostable, and it stores data in the proprietary cloud (at least exports are easy). But I understand that and accept it.
If I get a Macbook, an Android phone, and a Windows PC (a very popular combination of devices, btw), I honestly don't have energy to investigate how to inter-operate their passkey solutions. From the get go, I assume it's gonna be either impossible, or very hacky and fragile. Maybe I'm wrong, but last time I checked I simply walked away thinking "nope, no passkeys for me".