you can't just share passkey between your devices like you can with a password

Either you enroll a system that shares them between devices without the need for special interaction (password manager, iCloud etc) or you enroll each device separately into your account.

You can have more than one passkey for a service. This is a good thing.

I think that passkeys are simple, but no-one explains what they do and don't do in specific terms.

Someone compared it to generating private/public key pairs on each device you set up, which helps me a bit, but I recently set up a passkey on a new laptop when offered and it seemed to replace the option to use my phone as a passkey for the same site (which had worked), and was asking me to scan a QR code with my phone to set it up again.

So I don't know what went on behind the scenes there at all.

The only way I ever used passkeys is with bitwarden, and there you are sharing them between all bitwarden clients.

From my very limited experience, pass key allows to login faster and more reliable compared to letting bitwarden enter passwords and 2fa keys into the forms, but I still have the password and 2fa key stored in bitwarden as a backup in case passkey breaks.

To me, hardware tokens or passkeys are not there to replace passwords, but to offer a faster and more convenient login alternative. I do not want to rely on specific hardware (hardware token, mobile phone, etc.), because those can get stolen or lost.

Any of the multi-platform password managers that support pass keys will solve this.

You walk into the vault on every platform and your pass keys are magically shared between every platform you're logged into.

In any system that I've used pass keys for (which is every system that supports them), you can go into the password section and delete devices/passkeys.

To regenerate new passkeys they either support it directly in the spot where you deleted it or you log out log back in with username password and 2FA and it asks you again if you want to set up a passkey. I've not run into anything else.

you can’t just share passkey between your devices like you can with a password

You would just sign into your password manager or browser on both devices and have access to them?

Additionally, whatever app or service you're storing them in can provide sharing features, like how Apple allows you to share them with groups or via AirDrop.

there’s very little to no documentation about what you do if you lose access to the passkeys too.

If you lose your password, there are recovery options available on almost all accounts. Nothing about passkeys means the normal account recovery processes no longer apply.