Some questions on Yubikey, FIDO2, and passkeys

Some questions on Yubikey, FIDO2, and passkeys

From the documentation of Yubikey and passkeys, I got the impression that the passkey implementation is based on FIDO2, because they say the limit is 25 keys:

Currently, YubiKeys can store a maximum of 25 passkeys.

FIDO2 - the YubiKey 5 can hold up to 25 resident keys in its FIDO2 application.

In contrast:

FIDO U2F - similar to Yubico OTP, the U2F application can be registered with an unlimited number of services.

So is it correct that passkeys are implemented via FIDO2, or is it basically the same?

Also given the limit of 25 passkeys and the statement:

(...) passkeys in YubiKeys are bound to the YubiKey’s physical hardware where they can’t be copied.

• Is there a way to find out how many of the 25 passkey slots are used already?
• Is it possible to find out for which sites those allocated slots are being used for?
• Is there a way to delete or recycle any of those slots?