How is pre-master secret encrypted when ECC is used? The certificate used in https://de.wikipedia.org/wiki/Authenticated_Encryption
has


public key ECC
public key parameter ECDSA
key usage digital signature


It is used to setup a secure TLS connection.

So far I know, that a so called pre-master-secret is generated by client ant sent encrypted to the server, using its certificate.

But what crypto-primitive is used for encryption in that case? The public key is used for digital signature, ECDSA, but signing something is NOT encryption.

How then is the pre master secret encrypted?
How then is the pre master secret encrypted?


It isn't.
What you're looking at is the ECDHE key exchange for TLS.
The short description is that the server and client run a classic Diffie-Hellman  key exchange (on an elliptic curve) and use the resulting shared secret as the pre-master secret. This is unlike the RSA Key-Transport case where the client picks a pre-master secret and sends a public-key-encrypted copy to the server.



Now for the technical details (from RFC 5246: "The Transport Layer Security (TLS) Protocol Version 1.2"):
The client sends its list of supported cipher suites (in ClientHello) and the server responds with the cipher suite picked, which is TLS_ECDHE_ECDSA_CHACHA20_POLY1305 in this case (happens in ServerHello). In the same run the server sends its public, ephemeral ECDH key (in ServerKeyExchange) which is most likely a point of Curve25519. The client receives this and responds with his hown public key (again a point) and the client also computes the shared point and then uses the full x-coordinate as the pre-master secret. As soon as the server receives the client's public key it also computes the pre-master secret (ie the x-coordinate of the shared point).

Now both parties share the same pre-master secret, exchanged using ECDHE.