How Symmetric Key occurs in SSH process?
As I think that the Asymmetric key used for the only Authentication but my main problem is "How Server generates the Symmetric Key which will use to do secure communication"?
Thumb rule If there's Symmetric key involved, there must a secure key exchange algorithm.
In cryptography whenever they say Symmetric key is used to accomplish something, there's a need for secure transmission of that symmetric or Same key between the parties communicating. Such key exchange is done prior to any actual data being encrypted-transferred to arrive on an agreement which binds both parties to use certain parameters to be used in transmission of actual data. As this is seems to be critical process, cryptography provides certain key exchange algorithm and the most popular one is Diffie-Hellman. Be it SSL, SSH or any other protocol involving Symmetric key, DH is present.
ts;dr: Read about Diffie-Hellman