If you use a self-hosted password manager like bitwarden, then its there. If you use a file-based password manager like keepass, then you can use any syncing method you choose, and its no different from passwords.

not sure about every OS but the ios password app has a menu button to export them to other apps. I have exported mine to my own self hosted vaultwarden. took 2 minutes

You don’t, you can have multiple passkeys, one for each device.

Do you work in a blender? It's a pretty solid bit of plastic, should be fine in a pocket or wherever the device you're using it with is.

I'm using a FOSS password manager for like 6+ passkeys right now, on some of the biggest sites. So this is just demonstratably false.