How are used
ECDSA in combination with public key ? Usually these methods are used for establish a secret themselves, so why (and how) are used combined with public and private key couple in OpenPGP?
The proposed internet standard RFC-6637: Elliptic Curve Cryptography (ECC) in OpenPGP says in section 8:
The method is a combination of an ECC Diffie-Hellman method to establish a shared secret, a key derivation method to process the shared secret into a derived key, and a key wrapping method that uses the derived key to protect a session key used to encrypt a message. The One-Pass Diffie-Hellman method C(1, 1, ECC CDH) [NIST-SP800-56A] MUST be implemented with the following restrictions: the ECC CDH primitive employed by this method is modified to always assume the cofactor as 1, the KDF specified in Section 7 is used, and the KDF parameters specified below are used.
…but there is no word about public key receiver's use, it might be used to protect confidentiality.