The Nitrokey FIDO U2F is compliant to Web Authentication (WebAuthn) but it's not FIDO2. The main difference is that FIDO2 promises to replace passwords while FIDO U2F is supposed to be a 2nd factor (in addition to a password).
Nitrokey FIDO U2F is not a third party product but it's source is available here: https://github.com/Nitrokey/nitrokey-fido-u2f-firmware https://github.com/Nitrokey/nitrokey-fido-u2f-hardware